Refined routes to chlorin building blocks

ABSTRACT

A method of making chlorins comprises the steps of reacting (e.g. condensing) a dipyrrin western half intermediate with an eastern half intermediate to form a tetrahydrobilene, and then cyclizing the tetrahydrobilene to form a chlorin. Intermediates including tetrahydrobilenes useful in such reactions are also described.

RELATED APPLICATIONS

This application is a divisional of and claims priority to U.S. patent application Ser. No. 12/271,346, filed Nov. 14, 2008 now U.S. Pat. No. 7,678,900, now allowed, which is a divisional of U.S. patent application Ser. No. 11/072,196, filed Mar. 4, 2005, now U.S. Pat. No. 7,470,785, which is a divisional of U.S. patent application Ser. No. 10/140,654, filed May 8, 2002, now U.S. Pat. No. 6,946,552, and also claims the benefit of Provisional Application Ser. No. 60/289,985, filed May 10, 2001, the disclosure of each of which is incorporated by reference herein in its entirety.

STATEMENT OF FEDERAL SUPPORT

This invention was made with Government support under Grant No. GM36238 from the National Institutes of Health. The Government has certain rights to this invention.

FIELD OF THE INVENTION

The present invention concerns methods and intermediates useful for the synthesis of chlorins.

BACKGROUND OF THE INVENTION

A synthetic route that provides access to chlorin building blocks bearing substituents at the meso- and/or β-positions has recently been described (Strachan, et al. (2000) J. Org. Chem. 65:3160-3172; Balasubramanian, et al. (2000) J. Org. Chem. 65:7919-7929). In addition to selected patterns of functional group handles at the perimeter of the macrocycle, each chlorin bears a geminal dimethyl group to lock in the hydrogenation level yet lacks steric congestion or other unwanted functionality around the reduced ring. The synthesis involves the construction of an Eastern half and a Western half, which are joined to form the chlorin macrocycle in the final step (Scheme 1). This convergent coupling of the Eastern half and Western half is performed in a two-flask procedure involving acid-catalyzed condensation to give a dihydrobilene-a, followed by metal-mediated oxidative cyclization to give the chlorin. The Eastern half, a bromodipyrromethane-monocarbinol, is readily available by the acylation and bromination of a dipyrromethane at the 1- and 9-positions, respectively, followed by reduction. The Western half is a dihydrodipyrrin (1). The Western half has limited stability and generally must be prepared from the stable nitro-hexanone pyrrole precursor and used within a few days.

In our initial search for routes to a suitable Western half, we investigated the synthesis of a tetrahydrodipyrrin via an intermediate tetrahydrodipyrrin N-oxide (comprised of a pyrrole and a pyrroline N-oxide)(Strachan, et al. (2000) J. Org. Chem. 65:3160-3172). The formation of N-oxides by cyclization followed by deoxygenation affords a convenient entry to a number of heterocycles (Katritzky and Lagowski, Chemistry of the Heterocyclic N-Oxides, Academic Press: London and New York, 1971, pp. 166-231; Ochiai, E. Aromatic Amine Oxides, Elsevier: Amsterdam, 1967, pp. 184-209; Albini, A.; Pieta, S. Heterocyclic N-Oxides, CRC Press: Boca Raton, 1991, pp. 120-134). Indeed, pyrroline N-oxides played a central role throughout Todd's studies related to the synthesis of vitamin B₁₂ (Bonnett, et al. (1959) J. Chem. Soc. 2094-2102; Bonnett et al. (1959) J. Chem. Soc. 2102-2104; Bonnett, et al. (1959) J. Chem. Soc. 2105-2108; Brown et al. (1959) J. Chem. Soc. 2109-2116; Brown et al. (1959) J. Chem. Soc. 2116-2122; Clark, et al. (1959) J. Chem. Soc. 2123-2127; Bowering et al. (1963) Annalen 669:106-113; Brown, et al. (1965) J. Chem. Soc. 2337-2340; Brown et al. (1966) Tetrahedron, Suppl. 8, Part 1:15-26; Black, et al. (1976) J. Chem. Soc. Perkin Trans. I (18):1942-1943; Black, et al. (1976) J. Chem. Soc. Perkin Trans. I (18):1944-1950; Black, et al. (1976) J. Chem. Soc. Perkin Trans. I (18):1951-1954; Alderson et al. (1976) J. Chem. Soc. Perkin Trans. I (18):1955-1960). Battersby synthesized a tetrahydrodipyrrin N-oxide, converted it to the corresponding tetrahydrodipyrrin, and upon reaction with a 1-bromo-9-bromomethyldipyrrin in the presence of copper acetate obtained the copper chlorin in 6.9% yield (2.8 mg) (Battersby, et al. (1984) J. Chem. Soc. Perkin Trans. 1 (12):2725-2732). Though Battersby's pyrrole component was substituted with one ester and two alkyl groups, the route employed also proved suitable for our synthesis of a tetrahydrodipyrrin N-oxide incorporating an unsubstituted pyrrole unit (Strachan, et al. (2000) J. Org. Chem. 65:3160-3172). Thus, cyclization of a nitro-hexanone pyrrole (2) afforded the corresponding tetrahydrodipyrrin N-oxide (3), but we were unable to deoxygenate the cyclic nitrone and form the tetrahydrodipyrrin Western half (4) (Strachan, et al. (2000) J. Org. Chem. 65:3160-3172). We resorted to the cyclization of the nitro-hexanone pyrrole 2 with NaOMe/THF followed by TiCl₃ in NH₄OAc-buffered solution, forming the dihydrodipyrrin 1 directly (without isolating the N-oxide) in yields of 20-30% (Strachan, et al. (2000) J. Org. Chem. 65:316.0-3172; Balasubramanian, et al. (2000) J. Org. Chem. 65:7919-7929).

SUMMARY OF THE INVENTION

A first aspect of the present invention is a method of making a chlorin of Formula X:

wherein:

M is a metal, such as a metal selected from the group consisting of Cu, Zn, Mg, Pt, Pd, Sn and Al, or M is absent;

K¹, K² and K⁴ are independently selected atoms, such as atoms or hetero atoms independently selected from the group consisting of N, O, S, Se, Te, and CH (preferably not CH, and most preferably N);

K³ is N;

S¹, S², S³, S⁴, S⁵, S⁶, S⁷, S⁸, S⁹, S¹⁰, S¹¹, S¹², S¹³, and S¹⁴ are independently selected substituents, such as substituents selected from the group consisting of H, aryl, phenyl, cycloalkyl, alkyl, alkenyl, alkynyl, halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido, amido, and carbamoyl (e.g., H or alkyl). Preferably at least S⁸ and S¹⁴ are both not H (e.g., both are alkyl). In addition, from one to four of S¹, S², S³, S⁴ S⁵, S⁶, S⁷, S⁸, S⁹, S¹⁰, S¹¹, S¹², S¹³, and S¹⁴ may optionally be independently selected linking groups Q, such as linking groups Q are of the formula:

wherein:

n is from 0 or 1 to 5 or 10;

R³ may be present or absent;

R¹, R², and R³ are each independently selected from the group consisting of ethene, ethyne, aryl, and heteroaryl groups, which aryl and heteroaryl groups may be unsubstituted or substituted one or more times with H, aryl, phenyl, cycloalkyl, alkyl, alkenyl, alkynyl, halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido, amido, and carbamoyl; and

Y is a protected or unprotected reactive substituent, such as a reactive substituent selected from the group consisting of hydroxy, thio, seleno, telluro, ester, carboxylic acid, boronic acid, phenol, silane, sulfonic acid, phosphonic acid, alkylthiol, formyl, halo, alkenyl, alkynyl, haloalkyl, alkyl phosphonate, alkyl sulfonate, alkyl carboxylate, and alkyl boronate groups.

The method comprises oxidatively cyclizing a tetrahydrobilene of Formula XI:

wherein Z is selected from the group consisting of halo, alkoxy, and acyloxy (preferably in an organic solvent in the presence of a base, an oxidant and a metal salt MX_(n), where X is an anion, and n is 2-3, to produce a compound of Formula X above), and then optionally displacing metal M to create a free base chlorin.

A second aspect of the present invention is a method of making a tetrahydrobilene of Formula XI:

wherein: K¹, K² K³ and K⁴ are as described above; S¹, S², S³, S⁴ S⁵, S⁶, S⁷, S⁸, S⁹, S¹⁰, S¹¹, S¹², S¹³, and S¹⁴ are as described above; and Z is as described above. The method comprising condensing a compound of Formula WH with a compound of Formula EH

(preferably in an organic solvent in the presence of an acid) to form a tetrahydrobilene of Formula XI.

A third aspect of the present invention is a tetrahydrobilene of Formula XI:

wherein: K¹, K² K³ and K⁴ are as described above; S¹, S², S³, S⁴ S⁵, S⁶, S⁷, S⁸, S⁹, S¹⁰, S¹¹, S¹², S¹³, and S¹⁴ are as described above; and Z is as described above.

A fourth aspect of the present invention is a compound of Formula WH:

wherein:

K¹ is as described above; and S¹, S², S⁷, S⁸, S⁹, S¹², S¹³, and S¹⁴ are as described above (preferably subject to the proviso that S¹ and S² are not simultaneously alkyl).

A fifth aspect of the present invention is a method of making a compound of Formula WH:

wherein: K¹ is as given above, S¹, S², S⁷, S⁸, S⁹, S¹², S¹³, and S¹⁴ are as given above (not necessarily subject to the proviso noted immediately above). The method comprises reacting a compound of Formula III

in an organic solvent (preferably in the presence of Ti(0)) to form the compound of Formula WH.

A sixth aspect of the present invention is a compound according to Formula III:

wherein: K¹ is as given above; and S¹, S², S⁷, S⁸, S⁹, S¹², S¹³, and S¹⁴ are as given above (preferably subject to the proviso that S¹ and S² are not simultaneously alkyl).

A seventh aspect of the present invention is a method of making a compound of Formula III:

wherein: K¹ is as given above, and S¹, S², S⁷, S⁸, S⁹, S¹², S¹³, and S¹⁴ are as given above. The method comprises cyclizing a compound of Formula II:

(preferably in an organic solvent under reducing conditions, for example in the presence of zinc oxide and acetic acid) to produce a compound of Formula III.

An eighth aspect of the present invention is a compound of Formula II:

wherein K¹ is as given above, and S¹, S², S⁷, S⁸, S⁹, S¹², S¹³, and S¹⁴ are as given above.

A ninth aspect of the present invention is a compound of Formula X:

wherein:

M is as given above,

K¹, K² K³ and K⁴ are as given above;

S¹, S², S³, S⁴ S⁵, S⁶, S⁸, S⁹, S¹⁰, S¹¹, S¹², and S¹⁴ are as given above;

S⁷ and S¹³ are together ═O; and

from one to four of S¹, S², S³, S⁴ S⁵, S⁶, S⁸, S⁹, S¹⁰, S¹¹, S¹², and S¹⁴ are independently selected linking groups Q as given above. In a particular embodiment at least S¹ is a linking group Q, and in a particular embodiment S¹ is a halogen such as iodo.

The foregoing and other objects and aspects of the present invention are explained in greater detail in the drawings herein and the specification set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the effect of the concentration of TFA on the condensation of Eastern half (6a-OH) and Western half (4). The yield shown refers to the amount of chlorin formed upon treatment of the crude product to the oxidation conditions (analogous to a one-flask reaction) followed by UV-Vis spectroscopy.

FIG. 2 shows the effect of the duration of the condensation of Eastern half (6a-OH) and Western half (4) on the ultimate yield of chlorin. The yield shown refers to the amount of chlorin formed upon treatment of the crude product to the oxidation conditions (analogous to a one-flask reaction) followed by UV-Vis spectroscopy.

FIG. 3 shows the spectral evolution of the conversion of 7a to Zn-8a upon treatment to the oxidation conditions [tetrahydrobilene-a (7a) (10 mM), AgTf (3 mol equiv), Zn(OAc)₂ (15 mol equiv), 2,2,6,6-tetramethylpiperidine (15 mol equiv) in CH₃CN at reflux in air]. Time points shown were taken at 1 min, 3 min, 8 min, 15 min, 35 min, 1 h, 2 h, and 5 h. The absorption peaks at the Soret band and at the Q_(y)(0,0) band increase with time. The absorption peak at 505 nm rises and declines with time (from lowest to highest intensity the times were 1 min, 5 h, 2 h, 3 min, 1 h, 35 min, 8 rain, and 15 min).

FIG. 4 shows the yield of chlorin Zn-8a as a function of time upon treatment of 7a to the oxidation conditions [tetrahydrobilene-a (7a) (10 mM), AgTf (3 mol equiv), Zn(OAc)₂ (15 mol equiv), 2,2,6,6-tetramethylpiperidine (15 mol equiv) in CH₃CN at reflux in air]. The yield was determined by absorption spectroscopy (ε₆₀₉=43,600 M⁻¹ cm⁻¹) in toluene. Solid circles, THF; open circles, CH₃CN.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The term “substituent” as used in the formulas herein, particularly designated by S or S^(n) where n is an integer, in a preferred embodiment refer to electron-rich or electron-deficient groups (subunits) that can be used to adjust the redox potential(s) and/or spatial properties of the subject compound. Preferred substituents include, but are not limited to, H, aryl, phenyl, cycloalkyl, alkyl, alkenyl, alkynyl, halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, amido, and carbamoyl. In certain embodiments, a substituted aryl group is attached to a porphyrin or a porphyrinic macrocycle, and the substituents on the aryl group are selected from the group consisting of aryl, phenyl, cycloalkyl, alkyl, alkenyl, alkynyl, halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, amido, and carbamoyl. Additional substituents include, but are not limited to, 4-chlorophenyl, 4-trifluoromethylphenyl, and 4-methoxyphenyl. Preferred substituents provide a redox potential range of less than about 5 volts, preferably less than about 2 volts, more preferably less than about 1 volt.

The term “aryl” refers to a compound whose molecules have the ring structure characteristic of benzene, naphthalene, phenanthrene, anthracene, etc. (i.e., either the 6-carbon ring of benzene or the condensed 6-carbon rings of the other aromatic derivatives). For example, an aryl group may be phenyl (C₆H₅) or naphthyl (C₁₀H₇). It is recognized that the aryl group, while acting as substituent can itself have additional substituents (e.g. the substituents provided for S^(n) in the various formulas herein).

The term “alkyl” refers to a paraffinic hydrocarbon group, typically C1 to C4, which may be derived from an alkane by dropping one hydrogen from the formula. Examples are methyl (CH₃—), ethyl (C₂H₅—), propyl (CH₃CH₂CH₂—), isopropyl ((CH₃)₂CH—).

The term “alkenyl” refers to a hydrocarbon group, typically C2 to C4, derived from the corresponding alkyl and which contains at least one double bond (e.g., butadienyl).

The term “alkynyl” refers to a hydrocarbon group, typically C2 to C4, derived from the corresponding alkyl and which contains at least one triple bond (e.g., butadiynyl).

The term “halogen” refers to one of the electronegative elements of group VITA of the periodic table (fluorine, chlorine, bromine, iodine, astatine).

The term “perfluoroalkyl” refers to an alkyl group where every hydrogen atom is replaced with a fluorine atom.

The term “perfluoroaryl” refers to an aryl group where every hydrogen atom is replaced with a fluorine atom.

The term “pyridyl” refers to an aryl group where one CR unit is replaced with a nitrogen atom.

The term “sulfoxyl” refers to a group of composition RS(O)— where R is some alkyl, aryl, cycloalkyl, perfluoroalkyl, or perfluoroaryl group. Examples include, but are not limited to methylsulfoxyl, phenylsulfoxyl, etc.

The term “sulfonyl” refers to a group of composition RSO₂— where R is some alkyl, aryl, cycloalkyl, perfluoroalkyl, or perfluoroaryl group. Examples include, but are not limited to methylsulfonyl, phenylsulfonyl, p-toluenesulfonyl, etc.

The term “carbamoyl” refers to the group of composition R¹(R²)NC(O)— where R¹ and R² are H or some alkyl, aryl, cycloalkyl, perfluoroalkyl, or perfluoroaryl group. Examples include, but are not limited to N-ethylcarbamoyl, N,N-dimethylcarbamoyl, etc.

The term “amido” refers to the group of composition R¹CON(R²)— where R¹ and R² are H or some alkyl, aryl, cycloalkyl, perfluoroalkyl, or perfluoroaryl group. Examples include, but are not limited to acetamido, N-ethylbenzamido, etc.

The term “acyl” refers to an organic acid group in which the —OH of the carboxyl group is replaced by some other substituent (RCO—). Examples include, but are not limited to acetyl, benzoyl, etc.

In preferred embodiments, when a metal is designated by “M” or “M^(n)”, where n is an integer, it is recognized that the metal may be associated with a counterion.

As noted above, the present invention provides a method of making a chlorin of Formula X:

wherein:

M is a metal, such as a metal selected from the group consisting of Cu, Zn, Mg, Pt, Pd, Sn and Al, or M is absent;

K¹, K² and K⁴ are atoms, such as atoms or hetero atoms independently selected from the group consisting of N, O, S, Se, Te, and CH(CH is less preferred and N is most preferred);

K³ is N;

S¹, S², S³, S⁴ S⁵, S⁶, S⁷, S⁸, S⁹, S¹⁰ _(, S) ¹¹, S¹², S¹³, and S¹⁴ are independently selected substituents, such as substituents selected from the group consisting of H, aryl, phenyl, cycloalkyl, alkyl, alkenyl, alkynyl, halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido, amido, and carbamoyl.

In addition, from one to four of S¹, S², S³, S⁴ S⁵, S⁶, S⁷, S⁸, S⁹, S¹⁰, S¹¹, S¹², S¹³, and S¹⁴ may optionally be independently selected linking groups Q, for example linking groups Q of the formula:

wherein:

n is from 0 or 1 to 5 or 10;

R³ may be present or absent;

R¹, R², and R³ are each independently selected from the group consisting of ethene, ethyne, aryl, and heteroaryl groups, which aryl and heteroaryl groups may be unsubstituted or substituted one or more times with H, aryl, phenyl, cycloalkyl, alkyl, alkenyl, alkynyl, halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido, amido, and carbamoyl; and

Y is a protected or unprotected reactive substituent, for example a reactive substituent selected from the group consisting of hydroxy, thio, seleno, telluro, ester, carboxylic acid, boronic acid, phenol, silane, sulfonic acid, phosphonic acid, alkylthiol, formyl, halo, alkenyl, alkynyl, haloalkyl, alkyl phosphonate, alkyl sulfonate, alkyl carboxylate, and alkyl boronate groups.

The method comprises oxidatively cyclizing a tetrahydrobilene of Formula XI:

wherein Z is selected from the group consisting of halo, alkoxy, and acyloxy (preferably Br) (preferably in an organic solvent in the presence of a base, an oxidant and a metal salt MX_(n), where X is an anion, and n is 2-3, to produce a compound of Formula X above), and then optionally displacing metal M to create a free base chlorin. Any suitable solvent may be employed, including polar or nonpolar solvents and protic or aprotic solvents. Suitable bases that may be used in the reaction include but are not limited to piperidine and 2,2,6,6-tetramethylpiperidine. Any suitable oxidant may be used, including simply air, oxygen, silver iodate, etc. In a preferred embodiment the cyclizing step is carried out in the presence of a silver salt such as silver triflate. The optional step of displacing metal M may be carried out with an acid in accordance with known techniques.

Structurally in compounds described herein, S¹ and S⁵ may be independently selected trans-substituted linking groups Q, or S² and S⁶ may be independently selected trans-substituted linking groups Q¹ and Q².

A second aspect of the present invention is a method of making a tetrahydrobilene of Formula XI:

wherein K¹, K² K³ and K⁴ are as described above, and S¹, S², S³, S⁴ S⁵, S⁶, S⁷, S⁸, S⁹, S¹⁰, S¹¹, S¹², S¹³, and S¹⁴ are as described above; and Z is as described above. The method comprises condensing a compound of Formula WE with a compound of Formula EH

(preferably in an organic solvent in the presence of an acid) to form a tetrahydrobilene of Formula XI. Any suitable acid may be employed, such as a Bronsted or Lewis acid, one particular example being trifluoroacetic acid. The condensing step is preferably carried out under nonaqueous conditions. The organic solvent is preferably a polar or nonpolar aprotic solvent, such as acetonitrile, tetrahydrofuran or a mixture thereof.

The present invention further provides methods of making compounds of Formula WH, and the compounds so produced:

wherein K¹ is as given above, and S¹, S², S⁷, S⁸, S⁹, S¹², S¹³, and S¹⁴ are as given above. The method comprises reacting a compound of Formula III

in an organic solvent (preferably in the presence of Ti(0)) to form the compound of Formula WH.

A preferred group of compounds of Formula WH are those in which S¹ and S² are not simultaneously alkyl. One particularly preferred embodiment is compounds in which S¹ and S² are not simultaneously either alkyl or H (i.e., S¹ is not H or alkyl, and S² is not H or alkyl). In another particularly preferred embodiment, S¹ and/or S² are independently selected from the group consisting of Q, aryl, phenyl, cycloalkyl, alkenyl, alkynyl, halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido, amido, and carbamoyl. In another particular preferred embodiment, S¹ and/or S² is a linking group Q as described above.

A further aspect of the present invention is a method of making a compound of Formula III:

wherein: K¹ is as given above, and S¹, S², S⁷, S⁸, S⁹, S¹², S¹³, and S¹⁴ are as given above, and the compounds so produced. The method comprises cyclizing a compound of Formula II:

(preferably in an organic solvent and preferably under reducing conditions, such as in the presence of zinc and acetic acid) to produce a compound of Formula III.

Synthesis of Oxochlorins. Oxochlorins may be considered as a particular type of chlorin. Oxochlorins and chlorins have similar spectral properties but the oxochlorins are more resistant to oxidation than are chlorins. In fact, oxochlorins have oxidation potentials similar to those of porphyrins whereas chlorins have lower oxidation potentials than porphyrins. Thus, broadly speaking, an oxochlorin has the spectral properties of a chlorin and the oxidation properties of a porphyrin.

The time-honored method for forming oxochlorins employs treatment of a β-substituted porphyrin with OsO₄ forming the vicinal diol, which upon acid-catalyzed pinacol rearrangement yields the oxochlorin bearing a geminal dialkyl group (Chang, C. K.; Sotiriou, C. J. Heterocyclic Chem. 1985, 22:1739-1741). However, application of this approach to porphyrin building blocks bearing specific patterns of substituents at the perimeter of the macrocycle typically results in a mixture of oxochlorins (Osuka, A., et al., J. Am. Chem. Soc. 1996, 118:155-168). Battersby found that the attempted synthesis of a chlorin by reaction of an Eastern half and a Western half in the presence of copper acetate in air afforded the oxochlorin directly (Battersby, A. R., et al., J. Chem. Soc. Perkin Trans. 1 1984, 2725-2732). He stated in his paper that spectral monitoring indicated the chlorin was forming first and then undergoing oxidation to give the oxochlorin. The same reaction in the absence of air afforded the chlorin. While the oxochlorin was an undesired byproduct in Battersby's synthesis, the oxochlorin incorporated the keto functionality at a specific site and did not occur as a mixture of isomers.

In our synthesis, we employ zinc acetate in air to form the chlorin upon reaction of an Eastern half and a Western half. The chlorin so obtained can be oxidized to the oxochlorin, introducing the keto functionality adjacent to the geminal dimethyl group by oxidation of the isolated methylene group. Oxidation can be achieved by a variety of methods, including treatment with copper acetate and air, or oxidation with a number of reagents known to oxidize isolated methylene units. Such reagents include SeO₂, MnO₂, and CrO₃.

Statement of Utility. Chlorins (this term including oxochlorins) as described herein are useful for a variety of purposes known to those skilled in the art, including but not limited to the production of light harvesting arrays and solar cells as described in commonly owned, copending U.S. patent application Ser. No. 09/621,797 and Ser. No. 09/621,091, both filed Jul. 21, 2000, the disclosures of both of which are incorporated by reference herein in their entirety.

The following examples are provided to illustrate certain aspects of the invention, and are not to be construed as limiting thereof.

EXAMPLE 1 Refined Syntheses of Meso-Substituted Chlorins via Tetrahydrobilene Intermediates

In this Example, we describe a number of refinements that greatly facilitate access to chlorin building blocks. The synthesis of a new Western half (4) built around the tetrahydrodipyrrin nucleus is achieved by the deoxygenation of the corresponding N-oxide (3) under non-acidic conditions. The resulting tetrahydrobilene-a is stable, which enabled refinements to the conditions for both the condensation and the oxidation. We also have investigated a one-flask synthetic procedure for chlorin formation. Several new Eastern halves have also been prepared for enhanced solubility of the resulting chlorin building blocks in organic solvents. The meso-substituted chlorin building blocks are of interest in the synthesis of multi-chlorin arrays.

Synthesis of a Tetrahydrodipyrrin Western Half. The synthesis of the unsubstituted tetrahydrodipyrrin Western half 4 is shown in Scheme 2. The desired nitro-hexanone pyrrole 2 was prepared from pyrrole-2-carboxaldehyde by reaction with nitromethane, affording 2-(2-trans-nitrovinyl)pyrrole, followed by reduction with sodium borohydride and fluoride-mediated Michael addition of mesityl oxide. Reductive cyclization of 2 in the presence of Zn in acetic acid at room temperature as specified by Battersby ((1984) J. Chem. Soc. Perkin Trans. 1 (12):2725-2732) afforded the N-oxide 3 in <40% yield. A byproduct, observable by TLC analysis and estimated by ¹H NMR spectroscopy to be present in ˜3:2 ratio (3:byproduct), was isolated and assigned the structure shown for 5 (Scheme 2). Noteworthy features of the ¹H NMR spectrum of 5 include the following: (1) the presence of two resonances due to the pyrrole β-protons, in contrast to the three resonances due to the α- and β-protons exhibited by the N-oxide 3, and (2) disappearance of the singlets assigned to the amino and pyrrolic NH protons upon exchange with D₂O. The formation of such a cyclic byproduct cannot occur with pyrrole precursors bearing a full complement of alkyl substituents at the α- and β-positions (Battersby, et al. (1984) J. Chem. Soc. Perkin Trans. 1 (12):2725-2732). The same reaction performed in acetic acid diluted 1:1 with ethanol (to lessen the exotherm) and held at 0° C., with portion-wise addition of the Zn, resulted in a >9:1 ratio of 3:5. The residual byproduct 5 was readily removed by chromatography. Under these improved conditions the desired N-oxide 3 was isolated in 86% yield.

The next step involved deoxygenation of N-oxide 3 to give the tetrahydrodipyrrin 4. Numerous methods have been developed for the deoxygenation of heterocyclic N-oxides bearing various functional groups (Katritzky and Lagowski, Chemistry of the Heterocyclic N-Oxides, Academic Press: London and New York, 1971, pp. 166-231; Ochiai, E. Aromatic Amine Oxides, Elsevier: Amsterdam, 1967, pp. 184-209; Albini, A.; Pietra, S. Heterocyclic N-Oxides, CRC Press: Boca Raton, 1991, pp. 120-134). For the tetrahydrodipyrrin N-oxide, the ideal deoxygenation method must not be strongly acidic to avoid polymerizing the pyrrole unit, and must not cause reduction of the imine. The deoxygenation of 3 was examined with a variety of reagents (Zn, NaOH/methanol (den Hertog, et al. (1952) Rec. Trav. Chim. Pays-Bas. 71:1145-1151); Zn, aqueous NH₄Cl/THF (Jiu and Mueller (1959) J. Org. Chem. 24:813-818; Aoyagi, et al. (August 1997) Synthesis-Stuttgart 891-894); FeS O₄, aqueous NH₄Cl/CH₃CN (Talik and Plazek (1961) Roczniki Chem. 35:463-473); Mg, AcONH₄/methanol (Hahn and Lesiak (1985) Polish J. Chem. 59:627-629); Fe, AcONH₄/methanol (Hahn and Lesiak (1985) Polish J. Chem. 59:627-629); Ph₃P/toluene (Lu, et al. (1982) Synthesis-Stuttgart 185-186; Read, et al. (1983) Aust. J. Chem. 36:1227-1237); S/toluene (Relyea, et al. (1962) J. Org. Chem. 27:477-481); NaN₃/toluene (Di Nunno and Florio (1975) La Chim. E L'Ind (Milan) 57:243-244); Zn, NaI, Me₃SiCl/CH₃CN (Morita, et al. (1981) Chem. Lett. 921-924); at room temperature and at elevated temperatures but the N-oxide was resistant to deoxygenation with each of these reagents as determined by TLC or ¹H NMR analysis. In addition, NaBH₄/THF (Kawazoe and Tachibana (1965) Chem. Pharm. Bull. 13(9):1103-1107; Kawazoe and Araki (1968) Chem. Pharm. Bull. 16(5):839-847) reduced the N-oxide and the double bond affording several products. Treatment with TiCl₃ in a buffered NH₄OAc solution (Battersby, et al. (1984) J. Chem. Soc. Perkin Trans. 1 (12):2725-2732) gave the pyrrolo-atropine byproduct 5 (obtained previously but not identified (Strachan, et al. (2000) J. Org. Chem. 65:3160-3172). This failure is in contrast to the successful transformation (75% yield) achieved by Battersby and coworkers upon applying the same method to a tetrahydrodipyrrin N¹⁰-oxide which differed from 3 only in bearing one ester and two alkyl substituents on the pyrrole unit (Battersby, et al. (1984) J. Chem. Soc. Perkin Trans. 1 (12):2725-2732). Similar pyrrolo[3.2.1]azabicyclooctane products have been reported by rearrangement of tetrahydrodipyrrin derivatives (formed as intermediates in amine+keto-aldehyde condensations in studies of aging) and a plausible mechanism proposed involving electrophilic attack of the pyrroline imine at the pyrrole 3-position (Xu and Sayre (1999) Chem. Res. Toxicol. 12:862-868).

Application of a procedure for the deoxygenation of N-oxides with Ti(0)/THF at room temperature (Malinowski, M. (1987) Synthesis-Stuttgart (8):732-734) to 3 resulted in a variety of decomposition products as determined by ¹H NMR spectroscopy. Upon repeating the deoxygenation procedure with the addition of 2 molar equivalents of triethylamine to the Ti(0)/THF slurry (to neutralize the HCl liberated from the preparation of Ti(0)) prior to addition to the solution of 3 in THF, the desired deoxygenated product 4 was obtained as a crystalline solid following column chromatography. The tetrahydrodipyrrin 4 is quite stable, exhibiting negligible decomposition over one month upon storage at 0° C.

Investigation of the Synthesis of Chlorins. Our prior synthesis of chlorins involved (1) formation of the bromodipyrromethane-monocarbinol (Eastern half) by NaBH₄-reduction of the carbonyl group in the Eastern half precursor, (2) acid-catalyzed condensation of the Eastern half and the Western half to obtain the dihydrobilene-a, and (3) oxidative metal-mediated cyclization to give the chlorin (Strachan, et al. (2000) J. Org. Chem. 65:3160-3172; Balasubramanian, et al. (2000) J. Org. Chem. 65:7919-7929). All three steps were done in succession on the same day. However, condensation of 4 and 6a-OH (100 mM TFA in CH₃CN at room temperature) followed by oxidation under the conditions employed with 1 (excess AgIO₃, Zn(OAc)₂ and piperidine in anhydrous toluene at 80° C.) gave chlorin Zn-8a in only 7% yield. Similarly, reaction of 4 and 6c-OH afforded chlorin Zn-8c in 10% yield.

Studies of the Condensation. To understand whether the low yields of chlorin originated in the condensation process or the metal-mediated oxidative cyclization process, we sought to isolate the putative tetrahydrobilene-a, a linear tetrapyrrole derivative formed by condensation of the Western half and the Eastern half. The condensation of 4 and Eastern half 6a-OH (100 mM each) was performed at room temperature in CH₃CN containing 100 mM TFA (Eq 1). TLC analysis of the reaction mixture after 3 min. showed the presence of a new component. Chromatographic workup afforded the tetrahydrobilene-a 7a in 72% yield. The ¹H NMR spectrum of the tetrahydrobilene-a showed resonances characteristic of the respective Western half 4 and Eastern half precursor 6a. The three signals (7.75, 8.05, 9.29 ppm) assigned to the pyrrolic NH units each appeared as a broad multiplet. The appearance of multiple signals is not unexpected: the tetrahydrobilene-a contains three chiral centers and the isolated product can comprise up to eight diastereomers. Bilanes and derivatives are known to be sensitive to oxidation as well as attack by electrophilic species (e.g., acids), and to undergo intermolecular exchange of the pyrrole rings (Xue and Scott (September 1998) Tetrahedron Lett. 39(37):6651-6654). However, the tetrahydrobilene-a sample showed no decomposition upon storage as a solid for several months or in a solution of CDCl₃ for more than two weeks under argon in the refrigerator.

The surprising stability of the tetrahydrobilene-a and access to ˜500 mg quantities of this material enabled us to explore the reaction conditions of the separate condensation and oxidation steps. To modify the conditions of the condensation, the concentration of TFA and the reaction time were examined. The condensation of Western half 4 and Eastern half 6a-OH was performed for a given period of time; for quantitation, the resulting tetrahydrobilene-a 7a was subjected to the oxidative cyclization to form the chlorin. The yield of the chlorin Zn-8a was then determined by UV-vis spectroscopy. Refined conditions for the oxidative cyclization were employed (vide infra); these conditions employ AgTf, Zn(OAc)₂ and 2,2,6,6-tetramethylpiperidine in CH₃CN at reflux in air for 4.5 h.

The effect of the concentration of TFA (10 mM to 316 mM) was examined at room temperature for condensations of 30 min in CH₃CN. The results are shown in FIG. 1. The chlorin was formed in 51% yield upon use of 100 mM TFA. By contrast, the reaction of the dihydrodipyrrin (1) was performed with 10 mM reactants and 10 mM TFA, and yields of chlorin were ˜10-20%.

Two possible side reactions during acid catalyzed condensation were of considerable concern, including rearrangement of the Western half and acidolytic scrambling: (1) A likely mechanism for formation of byproduct 5 involves electrophilic attack of the positively charged imine (or nitrone) on the 3-position of the pyrrole. Treatment of 4 (100 mM) under the same acid catalysis conditions (100 mM TFA in CH₃CN at room temperature) gave a ratio of 95:5 for compounds 4:5 after 30 min but ˜20:80 after 12 h. The same reaction with 1 M TFA gave ˜15:85 after 30 min. These results show that the rearrangement of the Western half 4 to the byproduct 5 occurs under acid catalysis but the reaction is too slow to be competitive with the condensation with an Eastern half giving the tetrahydrobilene-a. (2) A key concern upon exposure of dipyrromethanes to acidic media is the occurrence of acidolysis yielding fragments that can recombine to form products with undesired substitution patterns (i.e., scrambling) (Littler, et al. (1999) J. Org. Chem. 64:2864-2872). We have found that dipyrromethane-carbinols (and the resulting porphyrinogens) are stable to modest concentrations of TFA in CH₃CN but that scrambling occurs within 30 min upon exposure to 100 mM TFA (Rao, et al. (2000) J. Org. Chem. 65:7323-7344; Rao et al. (2000) J. Org. Chem. 65:1084-1092). LD-MS analysis of the crude reaction mixture showed no scrambling with condensations employing 100 mM TFA. The absence of scrambling in the condensation of the Eastern and Western halves at higher acid concentration than in dipyrromethane condensations can be explained by consideration of the basicity of the different reactants. The weakly basic nature of the pyrrolic unit in a dipyrromethane (protonated 2-methylpyrrole has pK_(a)=−0.21) (Chiang and Whipple (1963) J. Am. Chem. Soc. 85:2763-2767) provides little capacity for buffering of the acid in reactions of the dipyrromethane. By comparison, the pyrroline motif in the Western half is basic (protonated 2,4,4-trimethyl-Δ¹-pyrroline has pK_(a)=7.6) (Bonnett, et al. (1959) J. Chem. Soc. 2087-2093). With equal concentrations of TFA and the Western half (100 mM each) the latter likely buffers the former, affording an effective acidity within the range that safely avoids acidolysis of the dipyrromethane or tetrahydrobilene-a species.

The time course of the condensation for reactions with 100 mM TFA at room temperature is shown in FIG. 2. These results show that the reaction is rapid and the yield changes only slightly over the course of 2 h, with a slight peak around 30 min. In summary, a condensation for 30 min at room temperature in CH₃CN containing 100 mM TFA results in the highest yield of chlorin.

Studies of the Metal-Mediated Oxidative Cyclization. While very little is known either about the intermediates or the sequence of events in this process, consideration of the formal transformations in the overall process guided our thinking in the development of reaction conditions. The role of the oxidant is to remove 6 electrons and 6 protons. The role of the base is to neutralize the acetic acid (2 equiv) formed upon zinc insertion and the HBr formed upon aromatization, and perhaps to facilitate imine-enamine tautomerization. The role of the metal complex is to template the electrocyclization. The role of the silver salt is to facilitate dehydrobromination. The overall process is expected to be complex. For example, oxidation is likely to occur in multiple steps, interspersed in which may be complexation and other processes.

The conversion of the tetrahydrobilene-a 7a to the zinc chlorin Zn-8a was examined under a variety of conditions. The conditions employed previously used AgIO₃ (15 mol equiv) Zn(OAc)_(Z) (15 mol equiv) and piperidine (15 mol equiv) in toluene at 80° C. for 2 h. The same conditions with 7a gave the desired zinc chlorin Zn-8a; the yield was low (7%) but a more severe problem was that iodinated chlorin byproducts were occasionally obtained. To avoid use of AgIO₃, we reexamined the use of high-potential quinones such as p-chloranil and DDQ, which are effective oxidants in the two-step one-flask synthesis of porphyrins (Lindsey, J. S. In The Porphyrin Handbook; Kadish, K. M.; Smith, K. M.; Guilard, R., Eds.; Academic Press: San Diego, Calif. 2000, Vol. 1, pp 45-118). We previously attempted to use p-chloranil but the chlorin product formed gave an incorrect molecule ion peak upon LD-MS analysis. The use of p-chloranil with the tetrahydrobilene-a gave two covalent chlorin-hydroquinone byproducts in greater yields than that of the desired chlorin. Quinones of lower potential such as duroquinone in the presence of additives (e.g., AgTf, AgBF₄) resulted in facile formation of the desired chlorin in yields of 60-70%, but we subsequently found that omission of the duroquinone caused no change in the yield. From this result, the oxidation was ascribed to O₂ in air. (It is likely that the oxidant in the previous reaction conditions employing AgIO₃ also was O₂.) This observation ultimately led to the following reaction conditions: AgTf (3 mol equiv), Zn(OAc)₂ (15 mol equiv) and 2,2,6,6-tetramethylpiperidine (15 mol equiv) in a solvent exposed to air. The following sections describe the iterative optimization process that led to these reaction conditions. The cleanliness of these reactions enabled the yield of chlorin to be determined by UV-vis spectroscopy.

We first examined the conversion of 7a into Zn-8a in ten solvents of diverse polarity and composition (Table 1). Acetonitrile, THF, and DMF were the best solvents (˜60% yield) but we focused on acetonitrile and THF for further studies because of their ease of handling. It is noteworthy that reagent-grade THF gave a slightly higher yield of chlorin (Zn-8a) compared to that with distilled THF.

TABLE 1^(a) Temp. Yield Solvent (° C.) (%)^(b) CH₃CN reflux 60 THF reflux 58 DMF 120° C. 57 1,2-dichloroethane reflux 44 DMSO 120° C. 42 Toluene reflux 39 Ethanol reflux 35 pyridine reflux 34^(c) CHCl₃ reflux 19 dioxane reflux 17 methanol reflux <1^(c) ^(a)All reactions were performed under the standard conditions employing the following components: tetrahydrobilene-a (7a) (10 mM), AgTf (3 mol equiv), Zn(OAc)₂ (15 mol equiv), 2,2,6,6-tetramethylpiperidine (15 mol equiv) under the specified conditions (solvent, temperature) exposed to air for 4.5 h. ^(b)Determined by absorption spectroscopy (ε₆₀₉ = 43,600 M⁻¹cm⁻¹) in toluene. ^(c)The long wavelength λ_(max) was at 612 nm.

Four amine bases were examined (Table 2) in THF or acetonitrile. Of the 4 amines examined, 2,2,6,6-tetramethylpiperidine gave the best results in both acetonitrile and THF (72%, 65%). Piperidine, which was used in the previous method, gave a low yield.

TABLE 2^(a) Yield (%)^(b) Base THF CH₃CN 2,2,6,6-tetramethylpiperidine 65 72 triethylamine 35 57 piperidine 41^(c) 31 2,5-di-tert-butylpyridine <1 <1 ^(a)All reactions were performed under the standard conditions employing the following components: tetrahydrobilene-a (7a) (10 mM), AgTf (3 mol equiv), Zn(OAc)₂ (15 mol equiv), specified base (15 mol equiv) at reflux in the specified solvent exposed to air for 4.5 h. ^(b)Determined by absorption spectroscopy (ε₆₀₉ = 43,600 M⁻¹cm⁻¹) in toluene. ^(c)The long wavelength λ_(max) was at 612 nm.

We examined the amount of AgTf required for the reaction (Table 3). When a large excess of AgTf was used, little chlorin Zn-8a was formed. The best result was obtained using 2 to 5 mol equiv of AgTf.

TABLE 3^(a) Mol equiv of AgTf Yield (%)^(b) 1 46 2 60 3 63 5 68 10  7 15 —^(c) ^(a)All reactions were performed under the standard conditions employing the following components: tetrahydrobilene-a (7a) (10 mM), AgTf (specified amount), Zn(OAc)₂ (15 mol equiv), 2,2,6,6-tetramethylpiperidine (15 mol equiv) at reflux in CH₃CN exposed to air for 4.5 h. ^(b)Determined by absorption spectroscopy (ε₆₀₉ = 43,600 M⁻¹cm⁻¹ in toluene). ^(c)The broad bands in the absorption spectrum precluded accurate yield determination.

To establish the absolute requirement for each of the reagents in the chlorin-forming reaction, omission experiments were performed (Table 4). In the absence of AgTf, chlorin was formed at only one-half to one-fourth of the normal level. No chlorin was observed in the absence of zinc acetate, and only a trace was obtained in the absence of 2,2,6,6-tetramethylpiperidine.

TABLE 4^(a) Yield (%)^(b) Conditions THF CH₃CN No omission (standard conditions) 58 60 Omission of AgTf 15 35 Omission of Zn(OAc)₂ — 0 Omission of 2,2,6,6-tetramethylpiperidine — <1 ^(a)All reactions were performed under the following conditions (with omission of the specified component): tetrahydrobilene-a (7a) (10 mM), AgTf (3 mol equiv), Zn(OAc)₂ (15 mol equiv), 2,2,6,6-tetramethylpiperidine (15 mol equiv) at reflux in the specified solvent exposed to air for 4.5 h. ^(b)Determined by absorption spectroscopy (ε₆₀₉ = 43,600 M⁻¹cm⁻¹) in toluene).

The concentration dependence of the reaction was examined by performing the reaction of the tetrahydrobilene-a 7a at 1, 3 or 10 mM and scaling the concentrations of the reagents linearly. The yield of chlorin was essentially constant over this 10-fold change in concentration.

The time course of the reaction was monitored by absorption spectroscopy. The spectral evolution of the oxidation process is shown in FIG. 3. Within one minute, a sharp absorption band is evident at 505 nm, which resembles that of a zinc-bis(dipyrrinato) chromophore (Granick and Gilder, In Advances in Enzymology; Nord, F. F. Ed.; Interscience: New York, 1947; Vol. 7, pp 358363; Johnson, et al. (1959) J. Chem. Soc. 3416-3424; Motekaitis and Martell (1970) Inorg. Chem. 9:1832-1839; March, et al. (1971) J. Chem. Soc. (A) 440-448; Murakami, et al. (1973) J. Chem. Soc. Dalton Trans. 1734-1737; Hill and Williamson (1985) J. Chem. Soc., Chem. Commun. 1228-1229; Brückner, et al. (1996) Can. J. Chem. 74:2182-2193) but such an assignment is not certain (see EXAMPLE 3). A zinc-dipyrrin could form by chelation of a partially oxidized product of the initial tetrahydrobilene-a. The peak at 505 nm increased as the reaction proceeded for 35 min and then decreased. The absorption spectra also show the appearance after 8 min of the characteristic chlorin peaks at 413 and 609 nm, which continue growing in over 1 h. After 1 h, more than 40% of tetrahydrobilene-a 7a had been converted into chlorin Zn-8a. The yield of chlorin as a function of time was readily assessed based on the intensity of the peak at 609 nm. The resulting yield versus time plot is shown in FIG. 4. The formation of the chlorin Zn-8a is essentially complete within 5 h.

In summary, the conditions for converting the tetrahydrobilene-a into the chlorin are as follows: tetrahydrobilene-a (10 mM), AgTf (3 to 5 mol equiv), Zn(OAc)₂ (15 mol equiv), and 2,2,6,6-tetramethylpiperidine (15 mol equiv) in acetonitrile at reflux in air for 4 to 6 h. These conditions were applied to a larger-scale synthesis of chlorin Zn-8a. Thus, 75.8 mg of tetrahydrobilene-a 7a was converted to 45 mg of chlorin Zn-8a in 62% yield.

One-Flask Procedure for Synthesis of Chlorins. The condensation yielding a tetrahydrobilene-a and subsequent oxidative cyclization yielding the chlorin has heretofore been implemented in a sequential two-flask procedure. The first step is an intermolecular reaction and should not require dilute conditions; the second step is an intramolecular cyclization and should proceed in greater yield under dilute conditions. Accordingly, we investigated a one-flask procedure employing 100 mM reactants in the condensation procedure and 10 mM reactants in the cyclization procedure. After condensation of Western Half (4) and Eastern half 6a-OH using 100 mM of TFA in acetonitrile, the reaction mixture was diluted 10-fold by addition of CH₃CN. The components for the oxidation were added [AgTf (3 mol equiv), Zn(OAc)₂ (15 mol equiv) and 2,2,6,6-tetramethylpiperidine (30 mol equiv rather than 15 equiv, to neutralize the TFA)] and the mixture was refluxed with exposure to air. After 18 h, the reaction mixture was chromatographed, affording chlorin Zn-8a in 31% yield (Eq 2). Although this one-flask approach is simple and readily implemented, the two-flask process affords a higher yield (45%) of chlorin.

Extension to the Synthesis of Meso-Substituted Chlorins. We investigated the synthesis of meso-substituted chlorins bearing two types of substituents: (1) strong electron-withdrawing groups and (2) synthetic handles at defined locations at the perimeter of the macrocycle. The former establish the scope of the methodology and the latter enable construction of chlorin-containing model systems in biomimetic or materials chemistry.

The synthesis of meso-substituted Eastern halves follows established methodology for the one-flask synthesis of meso-substituted dipyrromethanes (Littler, et al. (1999) J. Org. Chem. 64(4):1391-1396), monoacylation of a dipyrromethane (Rao, et al. (2000) J. Org. Chem. 65(4):1084-1092), and monobromination of a dipyrromethane (Strachan, et al. (2000) J. Org. Chem. 65:3160-3172). The precursors to meso-substituted Eastern halves 6b-e have been prepared (Scheme 3). The dipyrromethanes 9b-e (Rao, et al. (2000) J. Org. Chem. 65(4):1084-1092) were treated with ethylmagnesium bromide followed by pyridyl thioesters (10-12) (Strachan, et al. (2000) J. Org. Chem. 65:3160-3172; Goto, et al. (1980) Chem. Lett. (1):51-52) affording monoacyldipyrromethanes 13b-e in 54-60% yield. Bromination of the latter by treatment with NBS in THF at −78° C. afforded the desired precursor to the Eastern halves 6b-e in 65-84% yield.

While all of the tetrahydrobilenes (7a-f) were stable for a period of ≧1 month, the Eastern half precursors (1-bromo-9-aroyldipyrromethanes) exhibited a range of stability. Compounds 6a-c or 6d showed decomposition after a few days or one week, respectively, while 6e and 6f were stable for >1 month.

The synthesis of meso-substituted chlorin building blocks was carried out by the two-flask method as shown in Scheme 4. This route is to be compared with the previous route shown in Scheme 1. In each case, the new Western half 4 was employed. In these preparative syntheses, the oxidative cyclization process was monitored by absorption spectroscopy, showing the rise and fall of the intermediate (λ_(abs) ˜505 nm) and the formation of the chlorin (λ_(abs) ˜609 nm). In general the oxidative cyclization was nearly complete at 4-8 h but the reactions were often continued for up to 24 h.

The various meso-substituted chlorins prepared via this method are shown in Chart 1. The synthesis of chlorin Zn-8a provides a benchmark for yield comparisons. The reaction of 4 and 6a-OH afforded tetrahydrobilene 7a in 72% yield; oxidative conversion of the latter afforded Zn-8a in 62% yield. The reaction of the bis(pentafluorophenyl)-substituted Eastern half with 4 gave the tetrahydrobilene 7b in 32% yield and the chlorin Zn-8b in 38% yield. Several chlorin building blocks were prepared. The reaction of the Eastern half bearing a TMS-protected ester (6c-OH) with 4 gave the tetrahydrobilene 7c and chlorin Zn-8c in reasonable yields (55%, 41%). An iodo-substituted chlorin (Zn-8d) was prepared by reaction of the corresponding Eastern half 6d. We attempted to prepare a chlorin bearing a trimethylsilyl-protected ethyne by reaction of Eastern half 6e. The tetrahydrobilene 7e was formed smoothly. However, oxidative cyclization afforded the chlorin with the ethyne lacking the trimethylsilyl protecting group (Zn-7e). The synthesis of a similar trimethylsilyl-protected ethyne chlorin was attempted with oxidative cyclization of tetrahydrobilene 7f in THF rather than CH₃CN and again the deprotected ethynyl chlorin (Zn-8f) was obtained. In two cases a comparison was made of the oxidative cyclization in CH₃CN or in THF; in each case (7d, 7e) the yield of zinc chlorin was slightly higher in THF. Zinc chlorins Zn-7d and Zn-7e both incorporate the 3,5-di-tert-butyl group for increased solubility in organic solvents. In no case was scrambling yielding a mixture of chlorins detected.

It is noteworthy that in each synthesis the intermediate tetrahydrobilene-a was isolated in substantial quantities (73-460 mg), characterized, and found to be reasonably stable. The yields of chlorins were 2-4-fold greater than those obtained previously where comparisons could be made. For example, the prior synthetic method afforded chlorins Zn-8a, Zn-8b, or Zn-8c in yields of 10, 6, or 10% (see Experimental), respectively, to be compared with 45, 12, or 23% in the current method. Note that the deprotected ethynyl chlorin Zn-8f was obtained herein in 30% yield, in contrast to the 9% yield of the protected ethynyl chlorin by the previous method.

CHART 1

Stepwise Yields (%) Total Ar¹ Ar² Tetrahydrobilene 7 Chlorin Zn-8 Yield (%) a

72 62 45 b

32 38 12 c

55 41 23 d

40 40 (43) 16 e

59 R = TMS 27 (32) R = H 16 f

66 R = TMS (45) R = H 30 (yields in parentheses were obtained in THF rather than CH₃CN) General Experimental Methods.

¹H and ¹³C NMR spectra (300 MHz) were collected in CDCl₃ unless noted otherwise. Absorption spectra were obtained in toluene at room temperature. Chlorins were analyzed by laser desorption mass spectrometry (LD-MS) in the absence of a matrix (Fenyo, et al. (1997) J. Porphyrins Phthalocyanines 1:93-99; Srinivasan, et al. (1999) J. Porphyrins Phthalocyanines 3:283-291). Fast atom bombardment mass spectrometry (FAB-MS) data are reported for the molecule ion or protonated molecule ion. Pyrrole was distilled at atmospheric pressure from CaH₂. Melting points are uncorrected. p-Iodobenzaldehyde was obtained from Karl Industries, Inc. All other reagents and starting materials were obtained from Aldrich. Column chromatography was performed with flash silica (Baker). The reduction yielding the Eastern half was performed following a standard procedure for forming dipyrromethane-carbinols (Rao et al. (2000) J. Org. Chem. 65:1084-1092).

Solvents. THF was distilled from sodium benzophenone ketyl as required. Toluene was distilled from CaH₂. CH₃CN (Fisher certified A.C.S.) for use in the condensation process was distilled from CaH₂ and stored over powdered molecular sieves. Nitromethane was stored over CaCl₂. Anhydrous methanol was prepared by drying over CaH₂ for 12 h followed by distillation. Other solvents were used as received.

Non-commercial compounds. The compounds 2, 6a, 6b, 6f, 10, 12, 13a, 13b and 13f were prepared as described in the literature (Strachan, et al. (2000) J. Org. Chem. 65:3160-3172). The compounds 9a-d (Littler, et al. (1999) J. Org. Chem. 64:1391-1396) and 9e (Cho, et al. (1999) J. Org. Chem. 64:7890-7901) also were prepared as described in the literature.

1,3,3-Trimethyl-2,3,4,5-tetrahydrodipyrrin N¹⁰-oxide (3). Following a general procedure (Battersby, et al. (1984) J. Chem. Soc. Perkin Trans. 1 (12):2725-2732), to a vigorously stirred solution of 1-(2-pyrrolyl)-2-nitro-3,3-dimethyl-5-hexanone 2 (1.26 g, 5.29 mmol) in 25 mL of acetic acid and 25 mL of ethanol at 0° C., zinc dust (8.64 g, 132 mmol) was added slowly in small portions for 5 min. The reaction mixture was stirred at 0° C. for 15 min, and then was filtered through Celite. The filtrate was concentrated under high vacuum. The resulting brown solid was purified by column chromatography [silica; packed and eluted with ethyl acetate/CH₂Cl₂ (1:1), then eluted with CH₂Cl₂/methanol (9:1)] affording a brown oil that solidified to brownish crystals on standing at room temperature (943 mg, 86%): mp 85-87° C.; ¹H NMR δ 1.12 (s, 3H), 1.17 (s, 3H), 2.04 (s, 3H), 2.28 (d, J=17.6 Hz, 1H), 2.44 (d, J=17.6 Hz, 1H), 2.95-3.10 (m, 2H), 3.82-3.96 (m, 1H), 5.85-5.97 (m, 1H), 6.02-6.11 (m, 1H), 6.64-6.72 (m, 1H), 10.50-10.72 (br, 1H); ¹³C NMR δ 13.9, 23.4, 26.3, 28.5, 37.8, 47.7, 81.9, 107.0, 107.9, 118.1, 129.2, 147.3; FAB-MS obsd 206.1415, calcd 206.1419 (C₁₂H₁₈N₂O).

1,3,3-Trimethyl-2,3,4,5-tetrahydrodipyrrin (4). Following a general procedure for the deoxygenation of N-oxides (Malinowski, M. (August 1987) Synthesis-Stuttgart (8):732-734) with slight modification, TiCl₄ (2.87 mL, 26.2 mmol) was slowly added to a stirred solution of dry THF (60 mL) under argon at 0° C. To the resulting yellow solution was slowly added LiAlH₄ (665 mg, 17.5 mmol). The resulting black mixture was stirred at room temperature for 15 min and then triethylamine (23.0 mL, 164 mmol) was added. The black mixture was then poured into a solution of 3 (725 mg, 3.65 mmol) in dry THF (45 mL). The mixture was stirred for 30 min at room temperature and then water (45 mL) was added. The mixture was filtered. The filtrate was extracted with CH₂Cl₂. The organic layer was dried (Na₂SO₄) and evaporated under reduced pressure. The resulting yellow oil was purified by chromatography (silica, ethyl acetate) to give a pale yellow oil, which solidified to a pale yellow solid on cooling (448 mg, 65%): mp 53-54° C.; ¹H NMR δ 0.94 (s, 3H), 1.10 (s, 3H), 2.04 (m, 3H), 2.23-2.42 (m, 2H), 2.54-2.65 (m, 1H), 2.73-2.82 (m, 1H), 3.57-3.68 (m, 1H), 5.90-5.97 (m, 1H), 6.05-6.13 (m, 1H), 6.67-6.73 (m, 1H), 9.70-9.92 (br, 1H); ¹³C NMR δ 20.3, 22.6, 27.0, 27.9, 41.8, 54.1, 80.1, 105.1, 107.1, 116.3, 131.4, 174.6; FAB-MS obsd 191.1551, calcd 191.1548 (C₁₂H₁₈N₂)

Byproduct (5). To a vigorously stirred solution of 2 (2.00 g, 8.39 mmol) in 40 mL of acetic acid at room temperature, zinc dust (13.7 g, 210 mmol) was added all at once. The reaction mixture was stirred at room temperature for 1 h, and then was filtered through Celite. The filtrate was removed under vacuum and CH₂Cl₂ (100 mL) was added. The solution was washed with 10% aqueous Na₂CO₃ (100 mL). The organic layer was separated and chromatographed, affording 3 (143 mg, 8.2%). The aqueous layer was extracted with CH₂Cl₂ (3×100 mL) and the resulting organic layer was dried (Na₂SO₄) and concentrated to give a dark pink solid which still contained about 10% of the N-oxide (720 mg). Recrystallization (CHCl₃) afforded dark pink crystals (254 mg, 13%). mp 145° C.; ¹H NMR δ 0.95 (s, 3H), 1.21 (s, 3H), 1.50 (s, 3H), 1.59 (d, J=11.7 Hz, 1H), 1.74 (d, J=11.7 Hz, 1H), 1.83-1.97 (br, 1H), 2.64 (d, J=16.1 Hz, 1H), 2.86 (dd, J=16.1 Hz, 5.1 Hz 1H), 3.36 (d, J=5.1 Hz, 1H), 5.92-5.99 (m, 1H), 6.54-6.60 (m, 1H), 7.72-7.92 (br, 1H); ¹³C NMR δ 24.5, 27.3, 28.5, 34.3, 42.0, 58.6, 60.6, 65.4, 103.4, 116.0, 123.6, 129.2; FAB-MS obsd 191.1553, calcd 191.1548 (C₁₂H₂₀N₂).

Two-flask procedure for chlorin formation using AgIO₃, exemplified for Zn(II)-17,18-dihydro-18,18-dimethyl-5-(4-methylphenyl)-10-mesitylporphyrin (Zn-8a). Following a general procedure for chlorin formation (Strachan, et al. (2000) J. Org. Chem. 65:3160-3172; Balasubramanian, et al. (2000) J. Org. Chem. 65:7919-7929), to a solution of 6a (138 mg, 0.300 mmol) in 10 mL of anhydrous THF/methanol (4:1) was added a 10-fold excess of NaBH₄ (113 mg, 3.00 mmol). The reaction was monitored by TLC [silica, hexanes/ethyl acetate (5:1)] and upon completion was carefully quenched with cold water (50 mL), then extracted with distilled CH₂Cl₂ (3×20 mL). The combined organic layers were dried (K₂CO₃) and concentrated under reduced pressure without heating to afford the carbinol 6a-OH. The residue was dissolved in 3 mL of anhydrous CH₃CN. 1,3,3-trimethyl-2,3,4,5-tetrahydrodipyrrin (4) (56.5 mg, 0.300 mmol) was added followed by TFA (23 μL, 0.30 mmol). The solution was stirred at room temperature for 30 min. The reaction was quenched with 10% aqueous NaHCO₃ (50 mL) and extracted with distilled CH₂Cl₂ (3×20 mL) The combined organic layers were washed with water, dried (Na₂CO₃) and concentrated in vacuo without heating. The residue was dissolved in 30 mL of toluene, to which AgIO₃ (1.27 g, 4.50 mmol), Zn(OAc)₂ (825 mg, 4.50 mmol) and piperidine (445 μL, 4.50 mmol) were added. The reaction mixture was heated at 80° C. for 2 h. The reaction mixture was concentrated under reduced pressure. Chromatography of the residue [silica, hexanes/CH₂Cl₂ (2:1)] afforded a blue solid (12 mg, 7%). Analytical data are consistent with literature values (Strachan, et al. (2000) J. Org. Chem. 65:3160-3172).

Studies of the Western Half (4)+Eastern Half (6a-OH) Condensation. A sample of 6a (58.5 mg, 100 μmol) was reduced with NaBH₄ (37.8 mg, 1.00 mmol) in 2 mL of anhydrous THF/methanol (4:1). The resulting 6a-OH and 4 (18.8 mg, 100 μmol) were dissolved in 10 mL of anhydrous CH₂Cl₂, then 1 mL portions of the solution were placed in each of 10 vials (each vial contains 10 μmol of 6a-OH and 4). The solvent was evaporated and then 0.1 mL of CH₃CN containing the desired TFA concentration (10 to 316 mM) was added. After the desired reaction time (1 min to 2 h), the solution was diluted with 0.9 mL of CH₃CN [containing AgTf (7.7 mg, 30 μmol) and 2,2,6,6-tetramethylpiperidine (25.3 μl, 150 μmol)]. Then Zn(OAc)₂ (27.5 mg, 150 μmol) was added and the mixture was refluxed for 4.5 h. The reaction yield was determined by UV-vis spectroscopy. In the yield determinations, CH₃CN was added to each reaction mixture to bring the volume to 4.0 mL (thereby negating any possible error due to solvent evaporation during the reflux period). A 25 μL aliquot was then removed and transferred to a 4 mL cuvette (containing toluene). Quantitation was then based on the absorption at 609 nm (ε₆₀₉=43,600 M⁻¹ cm⁻¹).

1,3,3-Trimethyl-10-(4-methylphenyl)-15-mesityl-19-bromo-2,3,4,5-tetrahydrobilene-a (7a). Following a general procedure, treatment of 6a (585 mg, 1.00 mmol) with NaBH₄ (370 mg, 10.0 mmol) in 20 mL of anhydrous THF/methanol (4:1) afforded 6a-OH. The sample of 6a-OH was dissolved in 10 mL of anhydrous CH₃CN, then 4 (188 mg, 1.00 mmol) and TFA (77 μL, 1.0 mmol) were added. The reaction mixture was stirred at room temperature for 30 min. Then 10% aqueous NaHCO₃ (50 mL) was added and the mixture was extracted with distilled CH₂Cl₂ (3×20 mL). The combined organic layers were washed with water, dried (Na₂CO₃) and concentrated in vacuo without heating. The resulting brown solid was purified by chromatography [silica, hexanes/ethyl acetate (5:1), and then ethyl acetate] to give a brown solid (460 mg, 72%): mp 67-70° C.; ¹H NMR δ 0.90 (s, 3H), 1.07 (s, 3H), 1.91 (s, 3H), 2.04 (s, 6H), 2.25 (s, 3H), 2.26-2.30 (m, 2H), 2.31 (s, 3H), 2.48-2.60 (m, 1H), 2.66-2.72 (m, 1H), 3.52-3.63 (m, 1H), 5.28-5.32 (m, 1H), 5.67-5.81 (m, 6H), 6.00-6.04 (m, 1H), 6.81 (s, 1H), 7.06-7.11 (m, 4H), 7.67-7.79 (br, 1H), 7.99-8.13 (br, 1H), 9.23-9.32 (br, 1H); FAB-MS obsd 635.2749, calcd 635.2730 (C₃₈H₄₃BrN₄).

Studies of the Metal-Mediated Oxidative Cyclization. A solution of tetrahydrobilene 7a (63.5 mg, 100 μmol) in 10 mL of anhydrous CH₂Cl₂ was divided into 0.5 mL portions in each of 20 vials (each vial contains 5 μmol of 7a). For the study examining the effect of the reaction solvent, CH₂Cl₂ was then evaporated, and 0.5 mL of the solvent of interest was added. The corresponding reagents for the oxidation [2,2,6,6-tetramethylpiperidine (13 μL, 77 μmol) and Zn(OAc)₂ (14 mg, 76 μmol)] were added and the mixture was refluxed for 4.5 h. The yield of chlorin was determined by UV-vis spectroscopy. In the yield determinations, CH₂Cl₂ was added to each reaction mixture to bring the volume to 4.0 mL (thereby negating any possible error due to solvent evaporation during the reflux period). A 50 μL aliquot was then removed and transferred to a 4 mL cuvette (containing toluene). Quantitation was then based on the absorption at 609 nm (ε₆₀₉=43,600 M⁻¹ cm⁻¹).

Two-flask procedure for chlorin formation, exemplified for Zn(II)-17,18-dihydro-18,18-dimethyl-5-(4-methylphenyl)-10-mesitylporphyrin (Zn-8a). A solution of 7a (75.8 mg, 0.120 mmol) in CH₃CN (12 mL) was treated with Zn(OAc)₂ (328 mg, 1.79 mmol), AgTf (91.9 mg, 0.360 mmol) and 2,2,6,6-tetramethylpiperidine (300 μL, 1.79 mmol). The reaction mixture was refluxed for 24 h. The reaction mixture was concentrated under reduced pressure, and then the residue was chromatographed [silica, hexanes/CH₂Cl₂ (2:1)] to give a blue solid (45 mg, 62%). Analytical data are consistent with literature values (Strachan, et al. (2000) J. Org. Chem. 65:3160-3172).

One-flask procedure for chlorin formation, exemplified for Zn(II)-17,18-dihydro-18,18-dimethyl-5-(4-methylphenyl)-10-mesitylporphyrin (Zn-8a). Following a general procedure, treatment of 6a (231 mg, 0.500 mmol) with NaBH₄ (185 mg, 5.00 mmol) in 20 mL of anhydrous THF/methanol (4:1) afforded 6a-OH. The sample of 6a-OH was dissolved in 5 mL of anhydrous CH₃CN, then 4 (94 mg, 0.50 mmol) and TFA (39 μL, 0.50 mmol) were added. The reaction mixture was stirred at room temperature for 30 min, then the reaction mixture was diluted with 45 mL of CH₃CN. AgTf (385 mg, 1.50 mmol), Zn(OAc)₂ (1.38 g, 7.50 mmol) and 2,2,6,6-tetramethylpiperidine (2.53 mL, 15.0 mmol, 30 mol equiv) were added. The resulting mixture was refluxed for 18 h. The reaction mixture was concentrated under reduced pressure. The residue was chromatographed [silica, hexanes/CH₂Cl₂ (2:0], affording a blue solid (118 mg, 31%). Analytical data are consistent with literature values (Strachan, et al. (2000) J. Org. Chem. 65:3160-3172).

S-2-Pyridyl 3,5-di-tert-butylbenzothioate (11). To a stirred solution of 2-mercaptopyridine (2.78 g, 25.0 mmol) in CH₂Cl₂ (50 mL) was added a solution of 3,5-di-tert-butylbenzoyl chloride (6.31 g, 25.0 mmol) in CH₂Cl₂ (125 mL) over 10 min. After 5 h, TLC showed complete consumption of the 2-mercaptopyridine, then 2 N NaOH was added. The organic phase was isolated, washed with water, then dried (Na₂SO₄) and the solvent was removed to afford a white solid. The solid was recrystallized in hexane affording a white solid (5.05 g, 62%): mp 73-74° C.; ¹H NMR δ 1.36 (s, 18H), 7.31-7.36 (m, 1H), 7.67-7.70 (m, 1H), 7.75-7.80 (m, 2H), 7.85-7.88 (m, 2H), 8.66-8.69 (m, 1H); ¹³C NMR δ 32.0, 35.7, 122.6, 124.2, 128.9, 131.4, 136.9, 137.7, 151.1, 152.3, 152.5, 190.8; Anal. Calcd for C₂₀H₂₅NOS: C, 73.35; H, 7.69; N, 4.28. Found: C, 73.40; H, 7.75; N, 4.23.

1-(4-Methylbenzoyl)-5-{4-[2-(trimethylsilyl)ethoxycarbonyl]phenyl}dipyrromethane (13c). Following the general procedure (Rao, et al. (2000) J. Org. Chem. 65:1084-1092), EtMgBr (13.1 mL, 13.1 mmol), 1.0 M in THF) was added to a solution of 9c (2.00 g, 5.46 mmol) in dry THF (10 mL) at room temperature under argon. The mixture was stirred at room temperature for 10 min and then cooled to −78° C. A solution of S-2-pyridyl 4-methylbenzothioate (10) (1.25 g, 5.45 mmol) in dry THF (10 mL) was added. The reaction mixture was maintained at −78° C. for 10 min, then the cooling bath was removed. After 3 h, the reaction was quenched with 100 mL of saturated aqueous NH₄Cl. The reaction mixture was extracted with CH₂Cl₂, washed with water, and then dried (Na₂SO₄) and concentrated under reduced pressure to give a dark foam. Column chromatography [silica packed with hexanes/ethyl acetate (10:1), eluted with hexanes/ethyl acetate (5:1)] afforded a golden amorphous solid (1.42 g, 54%): mp 67-70° C.; ¹H NMR δ 0.07 (s, 9H), 1.11 (t, J=8.1 Hz, 2H), 2.42 (s, 3H), 4.39 (t, J=8.1 Hz, 2H), 5.62 (s, 1H), 5.95-5.99 (m, 1H), 6.06-6.10 (m, 1H), 6.12-6.16 (m, 1H), 6.64-6.68 (m, 1H), 6.77-6.81 (m, 1H), 7.16-7.30 (m, 4H), 7.67 (d, J=7.3 Hz, 2H), 7.89 (d, J=7.3 Hz, 2H), 8.52-8.68 (br, 1H), 10.30-10.42 (br, 1H); ¹³C NMR δ −0.8, 18.0, 22.2, 44.6, 63.9, 108.6, 109.0, 111.3, 118.7, 121.4, 128.9, 129.6, 129.7, 130.1, 130.4, 130.8, 131.6, 136.1, 141.4, 143.1, 146.5, 167.1, 185.3; Anal. Calcd for C₂₉H₃₂N₂O₃Si: C, 71.87; H, 6.65; N, 5.78. Found: C, 71.78; H, 6.61; N, 5.89.

1-(3,5-Di-tert-butylbenzoyl)-5-(4-iodophenyl)dipyrromethane (13d). Following the general procedure (Rao, et al. (2000) J. Org. Chem. 65(4):1084-1092), reaction of 9d (1.80 g, 5.10 mmol) and 11 (1.70 g, 5.10 mmol) followed by column chromatography [silica hexanes/ethyl acetate (5:1)] afforded a golden amorphous solid (1.63 g, 56%): mp 119-120° C.; ¹H NMR δ 1.34 (s, 18H), 5.51 (s, 1H), 5.93-5.97 (m, 1H), 6.07-6.14 (m, 2H), 6.57-6.63 (m, 1H), 6.73-6.77 (m, 1H), 6.92 (d, J=8.1 Hz, 2H), 7.49 (d, J=8.1 Hz, 2H), 7.60-7.67 (m, 3H), 8.57-8.70 (br, 1H), 10.38-10.52 (br, 1H); ¹³C NMR δ 32.1, 35.6, 44.4, 93.3, 108.1, 108.5, 109.1, 111.3, 118.1, 118.8, 121.8, 124.1, 126.7, 130.9, 131.9, 138.3, 141.4, 141.8, 151.5, 186.6; Anal. Calcd for C₃₀H₃₃IN₂O: C, 63.83; H, 5.89; N, 4.96. Found: C, 63.59; H, 5.95; N, 4.83.

1-(3,5-Di-tert-butylbenzoyl)-5-{4-[2-(trimethylsilyl)ethynyl]phenyl}dipyrromethane (13e). Following the general procedure (Rao, et al. (2000) J. Org. Chem. 65(4):1084-1092), reaction of 9e (2.00 g, 6.28 mmol) and 11 (2.06 g, 6.28 mmol) followed by column chromatography [silica hexanes/ethyl acetate (5:1)] afforded a golden amorphous solid (2.00 g, 60%): mp 108° C. (dec.) ¹H NMR δ 0.23 (s, 9H), 1.34 (s, 18H), 5.57 (s, 1H), 5.93-5.97 (m, 1H), 6.06-6.16 (m, 2H), 6.58-6.62 (m, 1H), 6.75-6.80 (m, 1H), 7.10 (d, J=8.1 Hz, 2H), 7.28 (d, J=8.1 Hz, 2H), 7.60-7.68 (m, 3H), 8.62-8.70 (br, 1H), 10.45-10.52 (br, 1H); ¹³C NMR δ 0.7, 32.1, 35.6, 44.7, 94.9, 105.6, 108.4, 108.9, 111.4, 118.8, 122.1, 122.5, 124.2, 126.7, 128.9, 131.3, 131.8, 132.8, 138.4, 142.1, 142.4, 151.5, 186.8; Anal. Calcd for C₃₅H₄₂N₂OSi: C, 78.60; H, 7.92; N, 5.24. Found: C, 78.75; H, 7.96; N, 5.20.

1-Bromo-9-(4-methylbenzoyl)-5-{4-[2-(trimethylsilyl)ethoxycarbonyl]phenyl}dipyrromethane (6c). Following the general procedure (Strachan, et al. (2000) J. Org. Chem. 65:3160-3172; Balasubramanian, et al. (2000) J. Org. Chem. 65:7919-7929), a solution of 13c (470 mg, 0.970 mmol) in 25 mL of dry THF was cooled to −78° C. under argon. NBS (173 mg, 0.970 mmol) was added, and the reaction mixture was stirred for 1 h at −78° C. Hexanes (50 mL) and water (50 mL) were added and the mixture was allowed to warm to room temperature. The organic phase was extracted with CH₂Cl₂ and dried (Na₂SO₄) and the solvent was removed under reduced pressure without heating. Column chromatography [silica; hexanes/ethyl acetate (4:1)] afforded a light brown powder (444 mg, 81%): mp 152° C. (dec.); ¹H NMR δ 0.07 (s, 9H), 1.11 (t, J=8.1 Hz, 2H), 2.16 (s, 3H), 4.39 (t, J=8.1 Hz, 2H), 5.57 (s, 1H), 5.89-5.93 (m, 1H), 6.02-6.06 (m, 1H), 6.09-6.13 (m, 1H), 6.76-6.80 (m, 1H), 7.17 (d, J=8.1 Hz, 2H), 7.23 (d, J=8.1 Hz, 2H), 7.60 (d, J=8.1 Hz, 2H), 7.84 (d, J=8.1 Hz, 2H), 9.10-9.26 (br, 1H), 10.82-10.94 (br, 1H); ¹³C NMR δ −0.9, 17.9, 22.2, 44.6, 63.8, 98.8, 105.0, 110.4, 110.8, 111.2, 122.1, 128.7, 129.5, 129.8; 130.1, 130.3, 131.7, 132.4, 141.2, 143.2, 145.9, 167.0, 185.7; Anal. Calcd for C₂₉H₃₁BrN₂O₃Si: C, 61.81; H, 5.54; N, 4.97. Found: C, 61.96; H, 5.53; N, 4.93.

1-Bromo-9-(3,5-di-tert-butylbenzoyl)-5-(4-iodophenyl)dipyrromethane (6d). Following the general procedure outlined for the synthesis of 6c, reaction of 13d (800 mg, 1.42 mmol) with NBS (253 mg, 1.42 mmol) followed by column chromatography [silica hexanes/ethyl acetate (4:1)] afforded a light brown powder (770 mg, 84%): mp 88-91° C.; ¹H NMR δ 1.34 (s, 18H), 5.51 (s, 1H), 5.89 (t, J=2.9 Hz, 1H), 6.03 (t, J=2.9 Hz, 1H), 6.11 (t, J=2.9 Hz, 1H), 6.77 (t, J=2.9 Hz, 1H), 6.96 (d, J=8.1 Hz, 2H), 7.51 (d, J=8.1 Hz, 2H), 7.62 (s, 3H), 8.91-9.00 (br, 1H), 10.59-10.72 (br, 1H); ¹³C NMR δ 32.1, 35.6, 44.5, 93.5, 98.8, 110.4, 111.1, 111.4, 122.2, 124.2, 126.9, 130.9, 132.0, 132.6, 138.2, 138.4, 140.7, 141.3, 151.6, 187.0; Anal. Calcd for C₃₀H₃₂BrIN₂O: C, 56.00; H, 5.01; N, 4.35. Found: C, 56.15; H, 5.19; N, 4.22.

1-Bromo-9-(3,5-di-tert-butylbenzoyl)-5-{4-[2-(trimethylsilyl)ethynyl]phenyl}dipyrromethane (6e). Following the general procedure outlined for the synthesis of 6c, reaction of 13e (1.00 g, 1.87 mmol) with NBS (333 mg, 1.87 mmol) followed by column chromatography [silica hexanes/ethyl acetate (5:1)] afforded a golden amorphous solid (914 mg, 80%): mp 118° C. (dec.); ¹H NMR δ 0.23 (s, 9H), 1.33 (s, 18H), 5.56 (s, 1H), 5.86-5.91 (m, 1H), 6.01-6.04 (m, 1H), 6.08-6.12 (m, 1H), 6.74-6.79 (m, 1H), 7.15 (d, J=8.1 Hz, 2H), 7.29 (d, J=8.1 Hz, 2H), 7.58-7.67 (m, 3H), 8.94-9.06 (br, 1H), 10.62-10.76 (br, 1H); ¹³C NMR δ 0.6, 32.1, 35.6, 44.8, 95.3, 98.7, 105.3, 110.3, 111.1, 111.4, 122.0, 122.8, 124.2, 126.7, 128.8, 131.9, 132.7, 132.9, 138.3, 141.2, 141.3, 151.5, 186.9; Anal. Calcd for C₃₅H₄₁BrN₂OSi: C, 68.50; H, 6.73; N, 4.56. Found: C, 68.48; H, 6.87; N, 4.47.

1,3,3-Trimethyl-10,15-bis(pentafluorophenyl)-19-bromo-2,3,4,5-tetrahydrobilene-a (7b). Following the general procedure, treatment of 6b (176 mg, 0.300 mmol) with NaBH₄ (113 mg, 3.00 mmol) in 10 mL of anhydrous THF/methanol (4:1) afforded 6b-OH. The reaction of 6b-OH and 4 (57 mg, 0.30 mmol) in 3 mL of anhydrous CH₃CN containing TFA (23 μL, 0.30 mmol) for 30 rain followed by the standard workup afforded a brown solid (73 mg, 32%): mp 56-58° C.; ¹H NMR δ 0.90 (s, 3H), 1.11 (s, 3H), 1.92 (s, 3H), 2.23-2.42 (m, 2H), 2.50-2.72 (m, 2H), 3.52-3.62 (m, 1H), 5.73-6.08 (m, 8H), 8.27-8.46 (br, 1H), 8.58-8.70 (br, 1H), 9.72-9.85 (br, 1H); Anal. Calcd for C₃₄H₂₅BrF₁₀N₄: C, 53.77; H, 3.32; N, 7.38. Found: C, 53.51; H, 3.41; N, 6.97; FAB-MS obsd 759.1181, calcd 759.1215 (C₃₄H₂₅BrF₁₀N₄).

Zn(II)-17,18-Dihydro-18,18-dimethyl-5,10-bis(pentafluorophenyl)porphyrin (Zn-8b). A solution of 7b (77.7 mg, 0.102 mmol) in CH₃CN (10 mL) containing the oxidative cyclization reagents was refluxed for 24 h. Standard workup and chromatography [silica, hexanes/CH₂Cl₂ (2:1)] gave a greenish blue solid (28.2 mg, 38%). Analytical data are consistent with literature values (Strachan, et al. (2000) J. Org. Chem. 65:3160-3172).

1,3,3-Trimethyl-10-(4-methylphenyl)-15-{4-[2-(trimethylsilyl)ethoxycarbonyl]phenyl}-19-bromo-2,3,4,5-tetrahydrobilene-a (7c). Following the general procedure, treatment of 6c (282 mg, 0.500 mmol) with NaBH₄ (189 mg, 5.00 mmol) in 30 mL of anhydrous THF/methanol (4:1) afforded 6c-OH. The reaction of 6c-OH and 4 (94 mg, 0.50 mmol) in 5 mL of anhydrous CH₃CN with TFA (39 μL, 0.50 mmol) for 30 min followed by the standard workup afforded a brown solid (203 mg, 55%): mp 81-83° C.; ¹H NMR δ 0.14 (s, 9H), 0.96 (s, 3H), 1.14 (s, 3H), 1.15 (t, J=8.1 Hz, 2H), 1.92-1.97 (m, 3H), 2.24-2.43 (m, 2H), 2.38 (s, 3H), 2.53-2.67 (m, 1H), 2.71-2.80 (m, 1H), 3.57-3.68 (m, 1H), 4.47 (t, J=8.1 Hz, 2H), 5.32-5.43 (m, 2H), 5.72-5.84 (m, 4H), 5.84-5.90 (m, 1H), 6.05-6.10 (m, 1H), 7.10-7.18 (m, 4H), 7.24-7.34 (m, 2H), 7.90-8.06 (m, 1H), 7.96-8.04 (m, 2H), 8.21-8.46 (m, 1H), 9.23-9.36 (m, 1H); FAB-MS obsd 737.2907, calcd 737.2886 (C₄₁H₄₉BrN₄O₂Si).

Zn(II)-17,18-Dihydro-18,18-dimethyl-5-(4-methylphenyl)-10-[4-[2-(trimethylsilyl)ethoxycarbonyl]phenyl]porphyrin (Zn-8c). A solution of 7c (110 mg, 0.150 mmol) in CH₃CN (15 mL) containing the oxidative cyclization reagents was refluxed for 14 h. Standard workup and chromatography [silica, hexanes/CH₂Cl₂ (1:2)] gave a blue solid (43 mg, 41%): ¹H NMR δ 0.18 (s, 9H), 1.12 (t, J=5.5 Hz, 2H), 2.03 (s, 6H), 2.66 (s, 3H), 4.52 (s, 2H), 4.65 (t, J=5.5 Hz, 2H), 7.48 (d, J=8.1 Hz, 2H), 7.94 (d, J=8.1 Hz, 2H), 8.13 (d, J=8.1 Hz, 2H), 8.31 (d, J=4.4 Hz, 1H), 8.34 (d, J=8.1 Hz, 2H), 8.42 (d, J=4.4 Hz, 1H), 8.57-8.70 (m, 6H); LD-MS obsd 710.78; FAB-MS obsd 712.2232, calcd 712.2212 (C₄₁H₄₀N₄O₂SiZn); λ_(abs) (toluene) 413, 609 nm.

1,3,3-Trimethyl-10-(3,5-di-tert-butylphenyl)-15-(4-iodophenyl)-19-bromo-2,3,4,5-tetrahydrobilene-a (7d). Following the procedure, treatment of 6d (322 mg, 0.500 mmol) with NaBH₄ (189 mg, 5.00 mmol) in 30 mL of anhydrous THF/methanol (4:1) afforded 6d-OH. The reaction of 6d-OH and 4 (94 mg, 0.50 mmol) in 5 mL of anhydrous CH₃CN containing TFA (39 μL, 0.50 mmol) followed by standard workup afforded a brown solid (163 mg, 40%): mp 70-73° C.; ¹H NMR δ 0.91 (s, 3H), 1.06 (s, 3H), 1.25 (s, 18H), 1.83-1.87 (m, 3H), 2.17-2.34 (m, 2H), 2.51-2.62 (m, 1H), 2.66-2.76 (m, 1H), 3.52-3.59 (m, 1H), 5.22-5.27 (m, 1H), 5.29-5.35 (m, 1H), 5.66-5.88 (m, 5H), 5.97-6.03 (m, 1H), 6.87-6.93 (m, 2H), 7.00-7.07 (m, 3H), 7.54-7.62 (m, 2H), 7.79-7.94 (m, 1H), 8.08-8.28 (m, 1H), 9.08-9.22 (m, 1H); Anal. Calcd for C₄₂H₅₀BrIN₄: C, 61.69; H, 6.16; N, 6.85. Found: C, 61.57; H, 6.21; N, 6.70; FAB-MS obsd 817.2317, calcd 817.2342 (C₄₂H₅₀BrIN₄).

Zn(II)-17,18-Dihydro-18,18-dimethyl-5-(3,5-di-tert-butylphenyl)-10-(4-iodophenyl)porphyrin (Zn-8d). A solution of 7d (81.7 mg, 0.100 mmol) in CH₃CN (10 mL) containing the oxidative cyclization reagents was refluxed for 20 h. Standard workup and chromatography [silica, hexanes/CH₂Cl₂ (1:2)] gave a blue solid (32 mg, 40%): ¹H NMR δ 1.49 (s, 18H), 2.03 (s, 6H), 4.51 (s, 2H), 7.72 (t, J=1.5 Hz, 1H), 7.80 (d, J=8.1 Hz, 2H), 7.89-7.93 (m, 2H), 8.01 (d, J=8.1 Hz, 2H), 8.35 (d, J=4.5 Hz, 1H), 8.44 (d, J=4.5 Hz, 1H), 8.58 (s, 1H), 8.61-8.63 (m, 2H), 8.65-8.69 (m, 2H), 8.70-8.74 (m, 1H); LD-MS obsd 791.60; FAB-MS obsd 792.1667, calcd 792.1689 (C₄₂H₄₁IN₄Zn); λ_(abs) (toluene) 412, 609 nm.

1,3,3-Trimethyl-10-(3,5-di-tert-butylphenyl)-15-{4-[2-(trimethylsilyl)ethynyl]phenyl}-19-bromo-2,3,4,5-tetrahydrobilene-a (7e). Following the general procedure, treatment of 6e (307 mg, 0.500 mmol) with NaBH₄ (189 mg, 5.00 mmol) in 30 mL of anhydrous THF/methanol (4:1) afforded 6e-OH. The reaction of 6e-OH and 4 (94 mg, 0.50 mmol) in 5 mL of anhydrous CH₃CN containing TFA (39 μL, 0.50 mmol) followed by the standard workup afforded a brown solid (233 mg, 59%): mp 61-63° C.; ¹H NMR δ 0.24 (s, 9H), 0.90 (s, 3H), 1.06 (s, 3H), 1.25 (s, 18H), 1.83-1.90 (m, 3H), 2.21-2.47 (m, 2H), 2.49-2.62 (m, 1H), 2.66-2.75 (m, 1H), 3.51-3.60 (m, 1H), 5.25-5.35 (m, 2H), 5.65-5.88 (m, 5H), 5.96-6.02 (m, 1H), 6.97-7.04 (m, 3H), 7.04-7.14 (m, 2H), 7.32-7.42 (m, 2H), 7.81-7.96 (m, 1H), 8.07-8.21 (m, 1H), 9.08-9.20 (m, 1H); FAB-MS obsd 787.3802, calcd 787.3771 (C₄₇H₅₉BrN₄Si).

Zn(II)-17,18-dihydro-18,18-dimethyl-5-(3,5-di-tert-butylphenyl)-10-(4-ethynyl]phenyl)porphyrin (Zn-8e). A solution of 7e (118 mg, 0.150 mmol) in CH₃CN (15 mL) containing the oxidative cyclization reagents was refluxed for 12 h. Standard workup and chromatography [silica, hexanes/CH₂Cl₂ (2:1)] gave a blue solid (28 mg, 27%). Deprotection of the trimethylsilylethyne apparently occurred under these conditions: ¹H NMR δ 1.49 (s, 18H), 2.02 (s, 6H), 3.26 (s, 1H), 4.50 (s, 2H), 7.71-7.74 (m, 1H), 7.81 (d, J=8.1 Hz, 2H), 7.90-7.94 (m, 2H), 8.03 (d, J=8.1 Hz, 2H), 8.35 (d, J=4.5 Hz, 1H), 8.45 (d, J=4.5 Hz, 1H), 8.58 (s, 1H), 8.59-8.62 (m, 2H), 8.63-8.68 (m, 2H), 8.72 (d, J=4.5 Hz, 1H); LD-MS obsd 689.45; FAB-MS obsd 690.2748, calcd 690.2701 (C₄₄H₄₂N₄a); λ_(abs) (toluene) 413, 609 nm.

1,3,3-Trimethyl-10-(4-methylphenyl)-15-{4-[2-(trimethylsilyl)ethynyl]phenyl}-19-bromo-2,3,4,5-tetrahydrobilene-a (7f). Treatment of 6f (309 mg, 0.600 mmol) with NaBH₄ (226 mg, 6.00 mmol) in 40 mL of anhydrous THF/methanol (4:1) afforded 6f-OH. The reaction of 61-OH and 4 (113 mg, 0.600 mmol) in 6 mL of anhydrous CH₃CN containing TFA (46 μL, 0.60 mmol) followed by the standard workup afforded a brown solid (273 mg, 66%): mp 77-79° C.; ¹H NMR δ 0.27 (s, 9H), 0.90 (s, 3H), 1.08 (s, 3H), 1.85 (s, 3H), 2.27-2.31 (m, 2H), 2.32 (s, 3H), 2.49-2.57 (m, 1H), 2.66-2.71 (m, 1H), 3.52-3.56 (m, 1H), 5.23-5.27 (m, 2H), 5.68-5.72 (m, 4H), 5.80-5.84 (m, 1H), 5.99-6.03 (m, 1H), 7.05-7.07 (m, 2H), 7.07-7.10 (m, 4H), 7.35-7.39 (m, 2H), 7.84-7.96 (m, 1H), 8.16-8.37 (m, 1H), 9.16-9.28 (m, 1H); Anal. Calcd for C₄₀H₄₅BrN₄Si: C, 69.65; H, 6.58; N, 8.12. Found: C, 69.17; H, 6.69; N, 7.74; FAB-MS obsd 689.2713, calcd 689.2675 (C₄₀H₄₅BrN₄Si).

Zn(II)-17,18-Dihydro-18,18-dimethyl-5-(4-methylphenyl)-10-(4-ethynyl]phenyl)porphyrin (Zn-8f). A solution of 7f (158 mg, 0.229 mmol) in THF (23 mL) containing the oxidative cyclization reagents was refluxed for 24 h. Standard workup and chromatography [silica, hexanes/CH₂Cl₂ (2:1)] gave a greenish blue solid (61 mg, 45%). Deprotection of the trimethylsilylethyne apparently occurred under these conditions: ¹H NMR δ 2.03 (s, 6H), 2.66 (s, 3H), 3.27 (s, 1H), 4.51 (s, 2H), 7.48 (d, J=8.1 Hz, 2H), 7.81 (d, J=8.1 Hz, 2H), 7.94 (d, J=8.1 Hz, 2H), 8.03 (d, J=8.1 Hz, 2H), 8.34 (d, J=4.5 Hz, 1H), 8.41 (d, J=4.5 Hz, 1H), 8.58 (s, 1H), 8.60-8.64 (m, 2H), 8.65 (d, J=4.5 Hz, 1H), 8.66 (s, 1H), 8.69 (d, J=4.5 Hz, 1H); LD-MS obsd 591.68; FAB-MS obsd 592.1620, calcd 592.1605 (C₃₇H₂₈N₄Zn); λ_(abs) 413, 609 nm.

Conclusions. We have refined a number of steps in the synthesis of meso-substituted chlorin building blocks. The prior synthesis established a general route to chlorins with defined patterns of functional group handles at the perimeter of the macrocycle. The weaknesses of the prior synthesis included (1) yields of chlorin in the 6-24% range and amounts of chlorin typically in the 5-20 mg range; (2) very short shelf life of the Western half (a dihydrodipyrrin); (3) inability to isolate, characterize, or quantitate the putative dihydrobilene-a intermediate; (4) the use of AgIO₃ in the oxidation method which upon prolonged exposure to the reaction mixture resulted in iodinated chlorin byproducts. The refined method described herein affords a stable Western half (a tetrahydrodipyrrin) in good yield. The Western half undergoes smooth condensation with the Eastern half, affording a stable tetrahydrobilene-a intermediate. Studies of the conversion of the latter to the chlorin revealed simple reaction conditions [AgTf, Zn(OAc)₂, and 2,2,6,6,-tetramethylpiperidine in CH₃CN at reflux exposed to air] that proceed in a clean manner and in good yield. A one-flask synthetic procedure for chlorin formation has been developed. Using the two-flask procedure, several zinc chlorins were synthesized in yields of 16-45% (based on the Eastern and Western half precursors) and in quantities of 27-118 mg. The isolation of the intermediate tetrahydrobilene-a in each case enabled determination of the yield of the condensation of Eastern and Western halves as well as of the oxidative cyclization of the tetrahydrobilene-a yielding the zinc chlorin.

EXAMPLE 2 Refined Synthesis of β-Substituted Chlorin Building Blocks

In this Example, we have employed this refined route to prepare a β-substituted chlorin that bears an ethyne group and an iodo group in diametrically substituted positions, as well as a 3,5-di-tert-butyl group at one meso position to impart increased solubility in organic solvents. This building block is ideally suited for use in the synthesis of linear multi-chlorin arrays analogous to our multi-porphyrin based molecular wires (Wagner and Lindsey (1994) J. Am. Chem. Soc. 116:9759-9760). The synthesis of such trans-β-substituted chlorin building blocks requires the synthesis of β-substituted Eastern and Western halves.

Synthesis of Substituted Tetrahydrodipyrrin Western Halves. A critical step in the synthesis of the unsubstituted tetrahydrodipyrrin Western half (4) is the deoxygenation of the N-oxide. A Western half bearing a synthetic handle at the β-position provides a convenient entry into β-substituted chlorins (Balasubramanian, et al. (2000) J. Org. Chem. 65:7919-7929). We recently developed a route to a (3-substituted dihydrodipyrrin Western half (Balasubramanian, et al. (2000) J. Org. Chem. 65:7919-7929), which was used herein with modification to obtain the β-substituted tetrahydrodipyrrin counterpart. The requisite iodophenyl-substituted nitro-hexanone pyrrole 14 was readily prepared from 3-(4-iodophenyl)pyrrole. Subsequent formylation affords two products due to substitution at either the α- or α′-position; a distinction between the two products is essential because the two patterns of substitution ultimately yield chlorin building blocks with substitution at different β-positions. Such distinction is difficult by ¹H NMR spectroscopy (Balasubramanian, et al. (2000) J. Org. Chem. 65:7919-7929). The remainder of the synthesis of 14 proceeds as for that of 1 (aldol condensation with nitromethane, reduction with NaBH₄, Michael addition with mesityl oxide). Reductive cyclization of 14 in the presence of Zn/AcOH gave the N-oxide 15 in 43% yield (Scheme 5). An X-ray structure obtained of 15 confirmed the substitution pattern, thereby eliminating any ambiguity concerning the NMR interpretation of the formylated pyrrole precursor (EXAMPLE 3). This structure also confirms the assigned substitution pattern of the dipyrromethane derivatives that serve as the Eastern half, because the same formylated pyrrole precursor to 14 is employed to prepare the β-substituted Eastern half (vide infra). Deoxygenation of the N-oxide 15 yielded the β-substituted tetrahydrodipyrrin Western half 16 in 38% yield (Scheme 5). Sonogashira coupling of 16 with (trimethylsilyl)acetylene afforded the trimethylsilylethynyl substituted Western half 17 in 76% yield.

Synthesis of Eastern Halves. The synthesis of the β-substituted Eastern half began in the same manner as our prior synthesis of β-substituted dipyrromethanes (Balasubramanian and Lindsey (1999) Tetrahedron 55:6771-6784). A BOC-protected dipyrromethane (18) was obtained from 2-formyl-3-(4-iodophenyl)pyrrole through the protection of the pyrrolic nitrogen, reduction of the aldehyde and condensation with pyrrole. Treatment of BOC-protected dipyrromethane with 3.0 equiv of EtMgBr in THF followed by 3,5-di-tert-butylbenzoyl chloride afforded the monoacylated dipyrromethane 19 in 45% yield (Scheme 6). In this acylation step, we employed 3,5-di-tert-butylbenzoyl chloride instead of p-toluoyl chloride to improve the solubility of the resulting chlorin in organic solvents. Removal of the BOC group under standard conditions (Hasun, et al. (1981) J. Org. Chem. 46:157-164) gave 3-(4-iodophenyl)-9-(3,5-di-tert-butylbenzoyl)dipyrromethane (20). Sonogashira coupling (Sonogashira, et al. (1975) Tetrahedron Lett. (50):4467-4470) of 20 with (trimethylsilyl)acetylene afforded trimethylsilylethynyl dipyrromethane 21 in 82% yield. Reaction of 21 with NBS at −78° C. gave the corresponding monobromo monoacyl dipyrromethane 22 in 72% yield. Compound 22 serves as the precursor to the Eastern half 22-OH.

New Chlorins. Chlorins bearing synthetic handles at defined locations at the perimeter of the macrocycle are valuable building blocks for the construction of model systems in biomimetic or materials chemistry. Porphyrinic pigments bearing an ethyne group and an iodo group at diametrically opposed positions are particularly attractive for the construction of linear multiporphyrin arrays (Wagner, et al. (1996) J. Am. Chem. Soc. 118:11166-11180). We have found that the extent of electronic communication between porphyrins of a given electronic composition depends on the site of attachment of a linker on the macrocycle (Strachan, et al. (1997) J. Am. Chem. Soc. 119:11191-11201; Yang, et al. (1999) J. Am. Chem. Soc. 121:4008-4018). Accordingly, the availability of chlorins with diverse substitution patterns would enable study of such electronic effects in chlorins, complementing our prior studies with porphyrins. For the construction of linear arrays, substitution at the 2- and 12-positions is quite attractive (Balasubramanian, et al. (2000) J. Org. Chem. 65:7919-7929). The β-substituted Eastern and Western halves are well suited for this application. The reaction of 16 and 22-OH was performed using the previous two-flask procedure, affording chlorin 23 in 7.8% yield (Scheme 7). ¹H NMR spectroscopy confirmed the expected structure. The geminal dimethyl groups of 23 resonate as a singlet at δ 1.97, and the CH₂ in the reduced pyrrole ring gives rise to a singlet downfield at δ 4.51. The four β-pyrrole hydrogens resonate in the region of δ 8.56-8.81. The eight hydrogens of the aryl rings in the β-substituents of the chlorin resonate in the region of δ 7.82-8.14 as two pairs of doublets.

General Experimental Methods.

¹H and ¹³C NMR spectra (300 MHz) were collected in CDCl₃ unless noted otherwise. Absorption spectra (Cary 3, 0.25 nm data intervals) were obtained at room temperature. Chlorins were analyzed by laser desorption mass spectrometry (LD-MS) in the absence of a matrix (Fenyo, et al. (1997) J. Porphyrins Phthalocyanines 1:93-99; Srinivasan et al. (1999) J. Porphyrins Phthalocyanines 3:283-291). Pyrrole was distilled at atmospheric pressure from CaH₂. Melting points are uncorrected. p-Iodobenzaldehyde was obtained from Karl Industries, Inc. All other reagents and starting materials were obtained from Aldrich. Chromatography was performed using flash silica (Baker) or alumina (Fisher A540, 80-200 mesh).

Solvents. THF was distilled from sodium benzophenone ketyl. Toluene was distilled from CaH₂. CH₂Cl₂ was distilled from CaH₂. The CH₂Cl₂ employed for extraction in the chlorin synthesis was distilled from CaH₂ and shaken with anhydrous Na₂CO₃ prior to use. CH₃CN (Fisher certified A.C.S.) was distilled from CaH₂ and stored over powdered molecular sieves. Nitromethane was stored over CaCl₂. Anhydrous methanol was obtained as follows. Methanol received was dried over CaH₂ for 12 h, and then was distilled. Other solvents were used as received.

Non-commercial compounds. The compounds 14 (Balasubramanian, et al. (2000) J. Org. Chem. 65:7919-7929) and 18 (Balasubramanian and Lindsey (1999) Tetrahedron 55:6771-6784) were prepared as described in the literature.

1,3,3-Trimethyl-7-(4-iodophenyl)-2,3,4,5-tetrahydrodipyrrin N¹⁰-oxide (15). A solution of 1-[3-(4-iodophenyl)pyrro-2-yl]-2-nitro-3,3-dimethyl-5-hexanone (14) (0.50 g, 1.14 mmol) in acetic acid (10 mL) was treated with zinc dust (1.74 g, 26.47 mmol) all-at-once and the mixture was stirred vigorously for 1 h at room temperature. The crude reaction mixture was filtered (Celite) and concentrated. The residue was dissolved in CH₂Cl₂, washed (10% aq Na₂CO₃), dried (Na₂SO₄) and evaporated to afford a yellow solid. Recrystallization from ethyl acetate gave a white powder (200 mg, 43% yield): mp 173-174° C.; ¹H NMR δ 0.98 (s, 3H), 1.17 (s, 3H), 2.10 (s, 3H), 2.36 (d, J=17.4 Hz, 1H), 2.52 (d, J=17.4 Hz, 1H), 3.07 (m, 2H), 3.96 (m, 1H), 6.20 (m, 1H), 6.74 (m, 1H), 7.08 (d, J=7.8 Hz, 2H), 7.67 (d, J=7.8 Hz, 2H), 11.31 (br, 1H); ¹³C NMR δ 13.2, 22.7, 23.5, 26.8, 37.8, 46.8, 81.0, 90.1, 107.8, 117.0, 120.6, 125.7, 130.2, 137.0, 137.3, 145.5; Anal. Calcd. for C₁₈H₂₁NIO₂: C, 52.95; H, 5.18; N, 6.86. Found: C, 52.91; H, 5.23; N, 6.78.

1,3,3-Trimethyl-7-(4-iodophenyl)-2,3,4,5-tetrahydrodipyrrin (16). Following a procedure for the deoxygenation of N-oxides (EXAMPLE 1), TiCl₄ (1.0 mL, 8.8 mmol) was slowly added to a stirred solution of dry THF (20 mL) under argon at 0° C. To the resulting yellow solution, LiAlH₄ (223 mg, 5.9 mmol) was added slowly and the black mixture was stirred at room temperature for 15 min, then triethylamine (7.7 mL) was added. The black mixture was then poured into a solution of 15 (500 mg, 1.22 mmol) in THF (20 mL). The mixture was stirred for 2 h at room temperature and then water (20 mL) was added. The mixture was filtered to remove inorganic materials. The filtrate was extracted (CH₂Cl₂). The organic layer was dried (Na₂SO₄) and evaporated under reduced pressure. The resulting brown oil was purified by chromatography (silica, ethyl acetate) to give a light brown oil which solidified on cooling (180 mg, 38%): mp 90-91° C.; ¹H NMR δ 0.92 (s, 3H), 1.08 (s, 3H), 2.05 (s, 3H), 2.34 (m, 2H), 2.59 (m, 1H), 2.92 (m, 1H), 3.64 (d, J=11.7 Hz, 1H), 6.25 (m, 1H), 6.73 (m, 1H), 7.13 (d, J=6.6 Hz, 2H), 7.66 (d, J=6.6 Hz, 2H), 10.30 (br, 1H); ¹³C NMR δ 21.2, 23.5, 27.0, 27.8, 42.6, 54.9, 80.5, 90.4, 108.4, 117.0, 120.3, 128.9, 130.6, 137.8, 137.9, 175.3; FAB-MS obsd 393.0841, calcd exact mass 393.0828 (C₁₈H₂₁IN₂).

1,3,3-Trimethyl-7-[4-[2-(trimethylsilyl)ethynyl]phenyl]-2,3,4,5-tetrahydrodipyrrin (17). Samples of 16 (450 mg, 1.15 mmol), Pd₂(dba)₃ (81 mg, 0.09 mmol), Ph₃As (219 mg, 0.72 mmol), and CuI (17 mg, 0.09 mmol) were added to a 50 mL Schlenk flask. The flask was evacuated and purged with argon three times. Then deaerated anhydrous THF/triethylamine (12 mL, 1:1) was added followed by (trimethylsilyl)acetylene (243 μL, 1.72 mmol). The flask was sealed, immersed in an oil bath (37° C.), and the mixture was stirred overnight. Then CH₂Cl₂ (30 mL) was added and the mixture was filtered (Celite) and washed (CH₂Cl₂). The filtrate was concentrated. The resulting residue was purified by flash chromatography (silica, ethyl acetate) to afford a dark brown viscous oil (316 mg, 76%): ¹H NMR δ 0.25 (s, 9H), 0.92 (s, 3H), 1.07 (s, 3H), 2.05 (s, 3H), 2.34 (m, 2H), 2.61 (m, 1H), 2.95 (m, 1H), 3.65 (d, J=11.7 Hz, 1H), 6.29 (m, 1H), 6.74 (m, 1H), 7.32 (d, J=7.8 Hz, 2H), 7.47 (d, J=7.8 Hz, 2H), 10.27 (br, 1H); ¹³C NMR δ 0.7, 21.1, 23.5, 27.1, 27.7, 42.6, 54.8, 80.5, 94.1, 106.3, 108.4, 117.0, 119.8, 120.9, 128.3, 129.3, 132.7, 138.7, 175.4; FAB-MS obsd 363.2271, calcd exact mass 363.2257 (C₂₃H₃₀N₂Si).

3-(4-Iodophenyl)-9-(3,5-di-tert-butylbenzoyl)-10-N-(tert-butoxycarbonyl)-dipyrromethane (19). A solution of 18 (896 mg, 2.0 mmol) in anhydrous THF (30 mL) under argon at 0° C. was treated slowly with EtMgBr (1M in THF, 6 mL, 6.0 mmol). The mixture was stirred for 10 min at 0° C. Then a solution of 3,5-di-tert-butylbenzoyl chloride (760 mg, 3.0 mmol) in anhydrous THF (5 mL) was added slowly and stirring was continued for 1.5 h at 0° C. The reaction was quenched with saturated aqueous NH₄Cl and extracted (CH₂Cl₂). The combined organic layers were washed with water and brine, dried (Na₂SO₄), and concentrated. The product was purified by flash column chromatography [silica, hexanes/ethyl acetate (4:1)] to yield a brown viscous oil which was cooled overnight in the refrigerator. A minimum amount of hexanes was added followed by sonication, affording a white solid (598 mg, 45%): mp 178-179° C.; ¹H NMR δ 1.35 (s, 18H), 1.58 (s, 9H), 4.30 (s, 2H), 5.97 (m, 1H), 6.26 (m, 1H), 6.73 (m, 1H), 7.11 (d, J=6.6 Hz, 2H), 7.30 (m, 1H), 7.60 (m, 1H), 7.70 (m, 2H), 7.72 (m, 2H), 9.92 (br, 1H); ¹³C NMR δ 25.8, 28.5, 32.0, 35.6, 85.5, 93.0, 110.1, 112.3, 120.4, 122.1, 123.9, 126.3, 127.1, 128.2, 131.2, 131.3, 135.4, 138.2, 138.6, 139.2, 150.5, 151.3, 185.7; Anal. Calcd for C₃₅H₄₁IN₂O₃: C, 63.25; H, 6.22; N, 4.21. Found: C, 64.03; H, 6.35; N, 4.15.

3-(4-Iodophenyl)-9-(3,5-di-tert-butylbenzoyl)dipyrromethane (20). A solution of 19 (600 mg, 0.90 mmol) in anhydrous THF (10 mL) under argon at room temperature was treated with methanolic NaOMe (146 mg, 2.70 mmol, in 1.0 mL of anhydrous methanol). After 10 min, the reaction was quenched with a mixture of hexanes and water (20 mL, 1:1) and extracted with ethyl acetate. The combined organic layers were washed with water and brine, dried (Na₂SO₄), and concentrated. The residue was recrystallized from ethanol to give a pale brown solid (430 mg, 84%): mp 202-203° C.; ¹H NMR δ 1.30 (s, 18H), 4.18 (s, 2H), 6.23 (m, 2H), 6.48 (m, 1H), 6.93 (m, 1H), 7.25 (d, J=8.1 Hz, 2H), 7.68 (m, 1H), 7.75 (m, 4H), 10.23 (br, 1H), 11.85 (br, 1H); ¹³C NMR δ 25.6, 31.9, 35.6, 91.1, 109.0, 111.2, 118.1, 121.6, 124.0, 124.2, 124.8, 127.0, 130.9, 131.5, 137.3, 138.0, 138.2, 141.5, 151.8, 187.5; Anal. Calcd for C₃₀H₃₃IN₂O: C, 63.83; H, 5.89; N, 4.96. Found: C, 63.74; H, 6.09; N, 5.00.

3-[4-[2-(Trimethylsilyl)ethynyl]phenyl]-9-(3,5-di-tert-butylbenzoyl)dipyrromethane (21). Samples of 20 (1.0 g, 1.77 mmol), Pd₂(dba)₃ (125 mg, 0.14 mmol), Ph₃As (334 mg, 1.09 mmol), and CuI (26 mg, 0.14 mmol) were added to a 50 mL Schlenk flask. The flask was evacuated and purged with argon three times. Then deaerated anhydrous THF/triethylamine (18 mL, 1:1) was added followed by (trimethylsilyl)acetylene (376 μL, 2.66 mmol). The flask was sealed, immersed in an oil bath (37° C.), and the mixture was stirred overnight (16-18 h). Then CH₂Cl₂ (30 mL) was added and the mixture was filtered (Celite) and washed (CH₂Cl₂). The filtrate was concentrated. The resulting residue was purified by flash chromatography [silica, hexanes/ethyl acetate (3:1)] to afford a yellow oil which solidified on cooling (780 mg, 82%): mp 126-127° C.; ¹H NMR δ 0.26 (s, 9H), 1.27 (s, 18H), 4.20 (s, 2H), 6.20 (m, 2H), 6.44 (m, 1H), 6.87 (m, 1H), 7.40 (d, J=8.1 Hz, 2H), 7.50 (d, J=8.1 Hz, 2H), 7.64 (m, 1H), 7.72 (m, 2H), 10.14 (br, 1H), 11.73 (br, 1H); ¹³C NMR δ 0.7, 25.7, 31.9, 35.5, 94.4, 106.2, 109.1, 111.1, 118.1, 120.4, 122.0, 123.9, 124.1, 124.9, 126.9, 128.5, 131.5, 132.7, 138.1, 138.2, 141.3, 151.7, 187.45; Anal. Calcd for C₂₈H₂₈N₂OSi: C, 78.60; H, 7.92; N, 5.24. Found: C, 78.09; H, 8.03; N, 5.13.

1-Bromo-3-[4-[2-(trimethylsilyl)ethynyl]phenyl]-9-(3,5-di-tert-butylbenzoyl)-dipyrromethane (22). A solution of 21 (100 mg, 0.19 mmol) in anhydrous THF (6 mL) was cooled to −78° C. under argon. Recrystallized NBS (33 mg, 0.19 mmol) was added and the reaction mixture was stirred for 1 h (−78° C.), then the mixture was quenched with a mixture of hexanes and water (20 mL, 1:1) and allowed to warm to 0° C. The aqueous portion was extracted with anhydrous ether and the combined organic layers were dried (K₂CO₃). The solvent was evaporated under vacuum without heating. Purification by flash chromatography [silica, hexanes/ether (2:1)] afforded a pale yellow solid (83 mg, 72%): mp 163-165° C. (dec.); ¹H NMR δ 0.27 (s, 9H), 1.30 (s, 18H), 4.20 (s, 2H), 6.08 (m, 1H), 6.23 (m, 1H), 6.94 (m, 1H), 7.36 (d, J=7.8 Hz, 2H), 7.52 (d, J=7.8 Hz, 2H), 7.66 (s, 1H), 7.74 (s, 2H), 10.75 (br, 1H), 12.14 (br, 1H); ¹³C NMR δ 0.7, 24.8, 31.2; 34.9, 94.0, 97.9, 105.2, 110.0, 110.7, 120.3, 123.1, 123.7, 123.8, 125.5, 126.5, 127.9, 130.8, 132.0, 136.3, 137.3, 140.5, 151.1, 187.3; Anal. Calcd for C₃₅H₄₁BrN₂OSi: C, 68.50; H, 6.73; N, 4.56. Found: C, 68.06; H, 6.64; N, 4.49; FAB-MS obsd 612.2184, calcd exact mass 612.2172 (C₃₅H₄₁BrN₂OSi).

Zn(II)-17,18-Dihydro-18,18-dimethyl-2-(4-iodophenyl)-5-(3,5-di-tert-butylbenzoyl)-12-[4-[2-(trimethylsilyl)ethynyl]phenyl]porphyrin (23). Following the two-flask procedure (EXAMPLE 1), to a solution of 22 (123 mg, 0.20 mmol) in anhydrous THF/methanol (7.5 mL, 4:1) was added excess NaBH₄ (100 mg, 2.60 mmol) in small portions at room temperature. The reaction was monitored by TLC [alumina, hexanes/ethyl acetate (1:1)]. Upon completion, the reaction mixture was quenched with cold water (10 mL), then extracted with distilled CH₂Cl₂ (3×25 mL). The combined organic layers were washed with brine (50 mL), dried (K₂CO₃) and concentrated in vacuo without heating to leave the resulting carbinol 22-OH in ˜1-2 mL of CH₂Cl₂. A solution of 1,3,3-trimethyl-7-(4-iodophenyl)-2,3,4,5-tetrahydrodipyrrin (16) (78 mg, 0.20 mmol) in a few milliliters of anhydrous CH₃CN was combined with the carbinol, then additional anhydrous CH₃CN was added to give a total of 20 mL of CH₃CN. The solution was stirred at room temperature under argon and TFA (20 μL, 0.26 mmol) was added. The reaction was monitored by TLC [alumina, hexanes/ethyl acetate (3:1)]; after 1 h the carbinol had disappeared. The reaction mixture was quenched with 10% aqueous NaHCO₃ and extracted with distilled CH₂Cl₂ (3×25 mL). The combined organic layers were washed with water and brine, then dried (Na₂SO₄). The solvent was removed in vacuo at room temperature. The residue was dissolved in 14 mL of anhydrous toluene under argon, then AgIO₃ (848 mg, 3.0 mmol), piperidine (300 μL, 3.0 mmol) and Zn(OAc)₂ (550 mg, 3.0 mmol) were added. The resulting mixture was heated at 80° C. for 3 h. The reaction was monitored by TLC [silica, hexanes/CH₂Cl₂ (1:1); showing a single green spot]. The mixture was cooled to room temperature, then passed through a short column (silica, CH₂Cl₂). The major fraction was concentrated and again chromatographed [silica, hexanes/CH₂Cl₂ (2:1 then 1:1)] to give a greenish blue solid (15 mg, 7.8%): ¹H NMR δ 0.35 (s, 9H), 1.51 (s, 18H), 1.97 (s, 6H), 4.51 (s, 2H), 7.42 (s, 1H), 7.82 (d, J=9.0 Hz, 2H), 7.87 (d, J=7.8 Hz, 2H), 7.96 (m, 2H), 8.03 (d, J=8.1 Hz, 2H), 8.14 (d, J=8.1 Hz, 2H), 8.56 (d, J=4.5 Hz, 1H), 8.66 (m, 2H), 8.72 (m, 2H), 8.81 (d, J=4.5 Hz, 1H), 9.61 (s, 1H); LD-MS obsd 964.60; FAB-MS obsd 964.2382, calcd exact mass 964.2376 (C₅₃H₅₃IN₄SiZn); λ_(abs) (toluene)/nm 417 (log ε=5.30, fwhm=20 nm), 629 (4.86); λ_(em) 634, 691 nm.

Conclusions. We have developed a tetrahydrodipyrrin Western half for the synthesis of chlorin building blocks. The new Western Half is obtained via the reductive cyclization of a nitro-hexanone substituted pyrrole followed by mild deoxygenation with a Ti(0) reagent. The tetrahydrodipyrrin is more stable than the dihydrodipyrrin used previously. The facile condensation of the Eastern half and Western half is performed at room temperature for a few minutes. The oxidative cyclization is performed at 80° C. for several hours in the presence of a zinc template. The tetrahydrobilene-a formed in the condensation of the Eastern and Western halves has been isolated and found to be reasonably stable. A one-flask synthetic procedure for chlorin formation has been developed. Several zinc chlorins were synthesized, with yields of 9 to 19% in the ring-forming step.

EXAMPLE 3 Isolation and Characterization of a 2-iodo-chlorin Byproduct

During our initial synthesis of a chlorin using the tetrahydrodipyrrin Western half, we employed the condensation and oxidation conditions (Strachan, et al. (2000) J. Org. Chem. 65:3160-3172) developed for the dipyrrin Western half. Thus, a reaction was performed of 4 and an iodo-phenyl substituted Eastern half (6g-OH) in acetonitrile containing 10 mM TFA followed by oxidative cyclization using AgIO₃, Zn(OAc)₂, and piperidine in toluene at 80° C. (Scheme 8). In addition to the desired chlorin (Zn-8h), we identified a byproduct upon examination of the LD-MS spectrum. The LD-MS spectrum shows peaks at m/z=694 and 820, the former of which corresponds to the desired chlorin Zn-8h. This Δm of 126 is ascribed to the presence of an iodo atom (note that the chlorin target molecule bears a single iodo group; the byproduct bears two iodo groups). However, only a single chlorin spot was observed upon TLC analysis. The absorption spectrum in the Q_(y)(0,0) region was somewhat broad. Upon demetalation of the zinc-chlorin material, TLC analysis showed the presence of two chlorin species. Isolation and characterization (particularly with ¹H NMR and NOE experiments) revealed that the iodination occurred regiospecifically at the 2-position. We note that this iodinated byproduct may have useful synthetic applications. The following experimental section describes the isolation and characterization data for the 2-iodo-chlorin byproduct.

Synthesis of 17,18-dihydro-18,18-dimethyl-5-(4-methylphenyl)-10-(4-iodophenyl)-porphyrin (Fb-8g) and 17,18-dihydro-18,18-dimethyl-5-(4-methylphenyl)-2-iodo-10-(4-iodophenyl)porphyrin (Fb-8h). Following the general procedure, treatment of 6 g (272 mg, 0.500 mmol) with NaBH₄ (114 mg, 3.01 mmol) in 7.5 mL of anhydrous THF/methanol (4:1) afforded 6g-OH. The residue was dissolved in 50 mL of anhydrous CH₃CN. 1,3,3-trimethyl-2,3,4,5-tetrahydrodipyrrin (4, 100 mg, 0.530 mmol) was added following TFA (38 μL, 0.50 mmol, 10 mM). The solution was stirred at room temperature for 30 min. The reaction was quenched with 10% aqueous NaHCO₃ (50 mL) and extracted with distilled CH₂CL₂ (3×50 mL). The combined organic layers were washed with water, dried (Na₂CO₃) and concentrated in vacuo without heating. The residue was dissolved in 50 mL of toluene, to which AgIO₃ (2.12 g, 7.50 mmol), Zn(OAc)₂ (1.38 g, 7.50 mmol) and piperidine (740 μL, 7.50 mmol) were added. The reaction mixture was heated at 80° C. exposed to air for 2 h. The reaction mixture was concentrated under reduced pressure. The residue was passed through a short silica gel column eluting with CH₂Cl₂. The major fraction was concentrated and again chromatographed on silica eluting with hexanes/CH₂Cl₂ (2:1) to afford a blue solid (66 mg). From the blue solid, 56 mg was taken and dissolved in 10 mL of CH₂Cl₂ and treated with TFA (310 μL, 4.04 mmol). The demetalation was complete in 1 h as confirmed by UV-Vis and TLC analyses. Then 70 mL of CH₂Cl₂ and 2 mL of triethylamine were added to the reaction mixture. The mixture was washed with saturated aqueous NaHCO₃ (100 mL×2) and dried (Na₂SO₄). TLC analysis [silica, hexanes/CH₂Cl₂ (1:1)] showed two components: a former component (R_(f)=0.72; Fb-8h) and a latter component (R_(f)=0.57; Fb-8g). The solvent was removed under vacuum. Chromatography of the residue [silica, hexanes/CH₂Cl₂ (3:1)] afforded Fb-8g (32 mg, 12%) and Fb-8h (19 mg, 6%). Analytical data of Fb-8g are consistent with the literature values (Strachan, et al. (2000) J. Org. Chem. 65:3160-3172). Fb-8h: ¹H NMR δ-1.66 (bs, 2H), 2.07 (s, 6H), 2.67 (s, 3H), 4.58 (s, 2H), 7.51 (d, J=8.1 Hz, 2H), 7.82 (d J=8.1 Hz, 2H), 7.96 (d, J=8.1 Hz, 2H), 8.03 (d, J=8.1 Hz, 2H), 8.40 (d, J=4.4 Hz, 1H), 8.48 (d, J=4.4 Hz, 1H), 8.70 (d, J=4.4 Hz, 1H), 8.77 (d, J=4.4 Hz, 1H), 8.88 (s, 1H), 8.95 (s, 1H), 8.97 (s, 1H); LD-MS obsd 757.24; FAB-MS obsd 759.0480, calcd 759.0482 (C₃₅H₂₈I₂N₄); λ_(abs) (toluene) 420, 512, 649 nm.

The foregoing is illustrative of the present invention, and is not to be construed as limiting thereof. The invention is defined by the following claims, with equivalents of the claims to be included therein. 

1. A compound of Formula WH:

wherein: X¹ is selected from the group consisting of N, O, S, Se, Te, and CH; S¹, S², S⁷, S⁸, S⁹, S¹², S¹³, and S¹⁴ are independently selected from the group consisting of H, aryl, phenyl, cycloalkyl, alkyl, alkenyl, alkynyl, halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido, amido, and carbamoyl; at least one of S¹ and S² is selected from the group consisting of Q, aryl, phenyl, cycloalkyl, alkenyl, alkynyl, halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido, amido, and carbamoyl; and wherein from one to two of S¹, S², S⁷, S⁸, S⁹, S¹², S¹³, and S¹⁴ may optionally be independently selected linking groups Q, wherein said linking groups Q are of the formula:

wherein: n is from 0 to 10; R³ may be present or absent; R¹, R², and R³ are each independently selected from the group consisting of ethene, ethyne, aryl, and heteroaryl groups, which aryl and heteroaryl groups may be unsubstituted or substituted one or more times with H, aryl, phenyl, cycloalkyl, alkyl, alkenyl, alkynyl, halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido, amido, and carbamoyl; and Y is a protected or unprotected reactive substituent selected from the group consisting of hydroxy, thio, seleno, telluro, ester, carboxylic acid, boronic acid, phenol, silane, sulfonic acid, phosphonic acid, alkylthiol, formyl, halo, alkenyl, alkynyl, haloalkyl, alkyl phosphonate, alkyl sulfonate, alkyl carboxylate, and alkyl boronate groups; subject to the proviso that S¹ and S² are not simultaneously alkyl.
 2. The compound according to claim 1, wherein S¹ is selected from the group consisting of Q, aryl, phenyl, cycloalkyl, alkenyl, alkynyl, halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido, amido, and carbamoyl.
 3. The compound according to claim 1, wherein S² is selected from the group consisting of Q, aryl, phenyl, cycloalkyl, alkenyl, alkynyl, halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido, amido, and carbamoyl.
 4. The compound according to claim 1, wherein S¹ is a linking group Q.
 5. The compound according to claim 1, wherein S² is a linking group Q.
 6. The compound according to claim 1, wherein K¹ is selected from the group consisting of N, O, S, Se and Te.
 7. The compound according to claim 1, wherein K¹ is N.
 8. The compound according to claim 1, wherein S¹, S², S⁷, S⁸, S⁹, S¹², S¹³, and S¹⁴ are all alkyl.
 9. The compound of claim 1, wherein said compound has the formula:


10. The compound of claim 1, wherein R¹, R², and R³ are each independently selected from the group consisting of ethene, ethyne, and aryl, which aryl and groups may be unsubstituted or substituted one or more times with H, aryl, phenyl, cycloalkyl, alkyl, alkenyl, alkynyl, halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido, amido, and carbamoyl. 